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Firefox 4, which shipped on March 22, 2011, enhances performance, adds more support for HTML5 and other evolving Web technologies, and further improves security. This article provides information about this release and what features are available for Web developers, add-on developers, and Gecko platform developers alike.

Features for web developers

Gecko now uses the HTML5 parser, which fixes bugs, improves interoperability, and improves performance. It also lets content embed SVG and MathML directly in the HTML markup.


Meet the HTML5 parser
A look at what the HTML5 parser means to you, and how to embed SVG and MathML into your content inline.
Forms in HTML5
A look at improvements to web forms in HTML5. Among these changes are added input types in the <input> element, data validation, and more.
HTML5 Sections
Gecko now supports the new HTML5 elements related to sections in a document: <article>, <section>, <nav>, <aside>, <hgroup>, <header> and <footer>.
HTML5 hidden attribute
This attribute, common to all elements, is used to hide content in a webpage that is not currently relevant to the user.
Other HTML5 elements
Gecko now also supports the following new HTML5 elements: <mark>, <figure>, and <figcaption>.
A guide to using the new WebSockets API for real-time communication between a web application and a server. Note that WebSockets as implemented in Firefox 4 is not compatible with the final standard, and should not generally be used.

Canvas improvements

The following changes were made to the CanvasRenderingContext2D interface to bring our <canvas> implementation closer to being in line with the specification:

  • Specifying a negative radius when calling arc() now correctly throws an INDEX_SIZE_ERR exception.
  • Specifying non-finite values when calling createLinearGradient() and createRadialGradient() now throws NOT_SUPPORTED_ERR instead of SYNTAX_ERR.
  • Setting miterLimit to a negative value no longer throws an exception; instead, it properly ignores non-positive values.
  • Setting lineWidth to a negative value no longer throws an exception; instead, it properly ignores non-positive values.
  • The putImageData() method now supports the optional parameters dirtyX, dirtyY, dirtyWidth, and dirtyHeight.

Miscellaneous HTML changes

  • <textarea> elements are now resizable by default; you can use the resize CSS property to disable this.
  • canvas.getContext and canvas.toDataURL no longer throw an exception when called with unrecognized arguments.
  • The <canvas> element now supports the Mozilla-specific mozGetAsFile() method, which lets you obtain a memory-based file containing an image of the canvas's contents. See HTMLCanvasElement for details.
  • canvas2dcontext.lineCap and canvas2dcontext.lineJoin no longer throw an exception when set to an unrecognized value.
  • canvas2dcontext.globalCompositeOperation no longer throws an exception when set to an unrecognized value, and no longer supports the non-standard darker value.
  • Support for the obsolete <spacer> element, which was absent in all other browsers, has been removed.
  • The <isindex> element, when created by calling document.createElement(), is now created as a simple element with no properties or methods.
  • Gecko now supports calling click() on <input> elements to open the file picker. See the example in the article Using files from web applications.
  • The <input> element supports a new mozactionhint attribute, which lets you specify the label for the enter key on virtual keyboards.
  • <script> elements inside <iframe>, <noembed>, and <noframes> elements now get executed, which they weren't in previous versions of Firefox. This is in compliance with the specification, and matches the behavior of other browsers.


CSS transitions
New CSS transitions support is available in Firefox 4.
Computed values in CSS
Support for -moz-calc has been added. This lets you specify <length> values as mathematical expressions.
Selector grouping
Support for :-moz-any to group selectors and factorize combinators.
Background image subrectangle support
The -moz-image-rect function makes it possible to use subrectangles of images as a background-image.
CSS touch properties
Support for touch properties is added. Details, and real article names, to come later.
Using arbitrary elements as CSS backgrounds
You can use the -moz-element CSS function and the document.mozSetImageElement() DOM function to use arbitrary HTML elements as backgrounds.
Privacy and the :visited selector
Changes have been made to what information can be obtained about the style of visited links using CSS selectors. This may affect some web applications.

New CSS properties

Property Description
-moz-font-feature-settings Lets you customized advanced features of OpenType fonts.
-moz-tab-size Specifies the width in space characters of a tab character (U+0009) when rendering text.
resize Lets you control the dimensions in which an element may be resized.

New CSS pseudo-classes

Pseudo-class Description
:-moz-handler-crashed Used to style elements whose plugins have crashed.
:-moz-placeholder Applied to placeholder text in form fields.
:-moz-submit-invalid Applied to the submit button on forms when one or more of the form's fields doesn't validate.
:-moz-window-inactive Applied to elements in inactive windows.
:invalid Automatically applied to <input> fields when their contents are invalid.
:optional Automatically applied to <input> fields that don't specify the required attribute.
:required Automatically applied to <input> fields that specify the required attribute.
:valid Automatically applied to <input> fields when their contents validate successfully.

New CSS pseudo-selectors

Pseudo-selector Description
:-moz-focusring Lets you specify the appearance of an element when Gecko believes it should have a focus indication rendered.

New CSS functions

Function Description
:-moz-any Lets you group selectors and factorize combinators.
-moz-calc Lets you specify <length> values as mathematical expressions.
-moz-element Lets you use an arbitrary element as a background for background-image and background.
-moz-image-rect Lets you use a subrectangle of an image as a background-image or background.

Renamed CSS properties

Old Name New Name Notes
-moz-background-size background-size The name -moz-background-size is no longer supported.
-moz-border-radius border-radius The old name is supported for a limited time to allow you time to update your sites. Rendering changes have also been made to match the latest version of the specification.
-moz-box-shadow box-shadow  

Miscellaneous CSS changes

  • The text-shadow property now caps the blur radius to 300px for sanity and performance reasons.
  • The overflow property no longer applies to table-group elements (<thead>, <tbody>, and <tfoot>).
  • The -moz-appearance property now supports the -moz-win-borderless-glass value, which applies a borderless Aero Glass look to an element.
  • The -moz-device-pixel-ratio media feature has been added, allowing the use of the device pixels per CSS pixel ratio to be used in Media Queries.
  • Gecko's handling of CSS units has been revised to better match other browsers, and to more accurately translate absolute lengths into screen pixel counts based on the device's DPI.

Graphics and video

The developing WebGL standard is now supported by Firefox.
Optimizing graphics performance
Tips and tricks for getting the most out of graphics and video performance in Firefox 4.
Support for WebM video
The new open WebM video format is supported by Gecko 2.0.
SVG animation with SMIL
Support for SMIL animation of SVG is now available.
Using SVG as images and as CSS backgrounds
You can now use SVG with the <img> element, as well as a CSS background-image.
Media buffered attribute support
The buffered attribute on <video> and <audio> elements is now supported, letting you determine which ranges of a media file have been buffered. The TimeRanges DOM interface has been implemented to support this.
Media preload attribute
The preload attribute from the HTML5 specification has been implemented, replacing the previously-implemented (and no longer supported) autobuffer attribute. This affects the <video> and <audio> elements, as well as the nsIDOMHTMLMediaElement interface.
SVG text positioning improvements
You can now specify lists for the values of the x, y, dx, and dy properties on SVG <text> and <tspan> elements. This lets you control the positioning of each character in a string individually.


JavaScript typed arrays
Support has been added for JavaScript typed arrays; this allows you to manipulate buffers containing raw data using native data types. Several APIs make use of this, including the File API, WebGL, and WebSockets.
Obtaining boundary rectangles for ranges
The Range object now has range.getClientRects() and range.getBoundingClientRect() methods.
Capturing mouse events on arbitrary elements
Support for the Internet Explorer-originated setCapture() and releaseCapture() APIs has been added. See bug 503943.
Manipulating the browser history
The existing document history object, available through the window.history object, now supports the new HTML5 pushState() and replaceState() methods.
Animations using MozBeforePaint
A new event has been added which, in concert with the window.mozRequestAnimationFrame() method and window.mozAnimationStartTime property, provides a way to create animations that are synchronized with one another.
Touch and multi-touch events
Support has been added for touch and multi-touch events.

HTML elements' DOM interfaces have changed

Several HTML elements have had their DOM interfaces changed to the ones required by the HTML5 specification, as shown below.

Interface in Firefox 3.6 Interface in Firefox 4 HTML Element
HTMLSpanElement HTMLElement <abbr>, <acronym>, <address>, <b>, <bdo>, <big>, <blink>, <center>, <cite>, <code>, <dd>, <dfn>, <dt>, <em>, <i>, <kbd>, <listing>, <nobr>, <plaintext>, <s>, <samp>, <small>, <strike>, <strong>, <sub>, <sup>, , <tt>, <u>, <var>, <xmp>
HTMLDivElement HTMLElement <noembed>, <noframes>, <noscript>
HTMLWBRElement HTMLElement <wbr>

Miscellaneous DOM changes


Content Security Policy (CSP)
Content Security Policy (CSP) is a Mozilla proposal designed to help web designers and server administrators specify how content on their web sites interacts. The goal is to help detect and mitigate attacks including cross-site scripting and data injection attacks.
HTTP Strict Transport Security
HTTP Strict Transport Security is a security feature that lets a web site tell browsers that it should only be communicated with using HTTPS, instead of using HTTP.
The X-FRAME-OPTIONS response header
The X-FRAME-OPTIONS HTTP response header introduced in Internet Explorer 8 is now supported by Firefox. This allows sites to indicate whether or not their pages can be used in frames, and if so, whether or not to restrict that to the same origin.
User Agent string changes
As a means to reduce the amount of data and entropy sent out in HTTP requests (see bug 572650), the crypto strength and language tokens have been removed from the user agent string.


For an overview of the changes implemented in JavaScript 1.8.5, see New in JavaScript 1.8.5. JavaScript in Firefox 4 will have additional adherence to the ECMAScript 5 standard.

Developer tools

Using the Web Console
The Web Console tool is a useful debugging aid for web developers and extension developers alike.

Note:The Error Console is disabled by default starting in Gecko 2.0. You can re-enable it by changing the devtools.errorconsole.enabled preference to true and restarting the browser.

Changes for Mozilla and add-on developers

For helpful tips on updating existing extensions for Firefox 4, see Updating extensions for Firefox 4. There are several key changes that break compatibility with existing add-ons, so be sure to read that article.

If you're a theme developer, you should read Theme changes in Firefox 4 to understand some critical changes you'll need to be aware of.

JavaScript code modules

The Services.jsm code module provides getters that make it easy to obtain references to commonly-used services, such as the preferences service or the window mediator, among others.
JS-ctypes API
The JS-ctypes API makes it possible to call C-compatible foreign library functions without using XPCOM.
Add-ons Manager
The new Add-ons Manager provides information about installed add-ons, support for managing them, and provides ways to install and remove add-ons.
The new popup notifications module makes it easy to present attractive, non-modal notifications to the user. You can see how to use this API in Using popup notifications.
Loading code modules from chrome: URLs
You can now load JavaScript code modules using chrome: URLs, even inside JAR files.
The DownloadLastDir.jsm code module provides the gDownloadLastDir global variable, which contains a string you can use to learn the path of the directory into which the last download occurred. This module handles issues related to private browsing for you.
Measuring performance using the PerfMeasurement.jsm code module
The PerfMeasurement.jsm code module provides an API to measure CPU-level performance data in JavaScript code.

Miscellaneous changes to code modules

DOM changes

A new type of worker for privileged code; this lets you use things like js-ctypes from workers in extensions and application code.
Touch events
Support for (non-standard) touch events has been added; these let you track multiple fingers moving on a touch screen at the same time.

Other DOM changes


Changes to the tabbrowser element

Several changes were made to the tabbrowser element that impact extensions that interact with tabs. In addition to supporting app tabs, these changes also change the tab bar into a standard toolbar, which lets the user drag toolbar buttons into it.

  • The TabClose, TabSelect, and TabOpen events no longer bubble up to the tabbrowser element (gBrowser). Event listeners for those events should be added to gBrowser.tabContainer rather than to gBrowser directly.
  • The tab context menu is no longer an anonymous child of the tabbrowser. It can therefore be overlaid directly with XUL overlays. It can also be accessed more directly in JavaScript via gBrowser.tabContextMenu. See this blog post for more details.
  • The new visibleTabs property was added to let you get an array of the currently visible tabs; this lets you determine which tabs are visible in the current tab set. This is used by Firefox Panorama, for example.
  • Added the new showOnlyTheseTabs method; this is used by Firefox Panorama.
  • Added the new getIcon method, which lets you get a tab's favicon without having to pull up the browser element.
  • Added the new tabbrowser.tabs property, which lets you easily get a list of the tabs in a tabbrowser element.
  • The new pinTab and unpinTab methods let you pin and unpin tabs (that is, switch them between being app tabs and regular tabs).
  • Added the getTabModalPromptBox method and tabmodalPromptShowing attribute to the tabbrowser to support tab-modal alerts.

Changes to popups

  • The popup element is no longer supported; you should use menupopup instead. (If you continue using popup, you will encounter glitches, since the element has no special meaning anymore. For example, menuseparator can appear transparent when used in a popup.)
  • The menupopup XUL element now has a triggerNode property, which indicates the node on which the event occurred that caused the popup to open. This also required the addition of a trigger event parameter to the openPopup method. Also, the anchorNode property has been added; it returns the anchor specified when the popup was created.
  • The panel element now offers fade and flip attributes, which are used to configure the behavior of new "arrow" style notification panels.

Remote XUL support removed

Remote XUL is no longer supported; this affects XUL documents being served through HTTP; also, you can no longer load XUL documents using file:// URLs unless you create the preference dom.allow_XUL_XBL_for_file and set it to true. There is, however, a whitelist feature that can be used to allow specific domains to load remote XUL. The Remote XUL Manager extension lets you manage this whitelist.

Miscellaneous XUL changes

  • The readonly attribute now correctly works for XBL fields.
  • The resizer element now lets you use the element attribute to specify an element to resize, instead of resizing the window.
  • The resizer element now has an type attribute that lets you specify that the resizer is for a window instead of an element, to prevent the window resizer from being drawn twice.
  • The "active" attribute no longer gets set on active XUL windows. Instead, you can use the new :-moz-window-inactive pseudoclass in order to assign different styles to background windows.
  • The emptytext attribute is now deprecated; you should use placeholder instead.
  • The window element now offers a accelerated attribute; when true, the hardware layer manager is permitted to accelerate the window.
  • The stack element now supports the bottom and right attributes.
  • Events are now fired during toolbox customization, allowing you to detect changes to toolbars.
  • The alternatingbackground attribute for tree elements is no longer supported; you can use the :-moz-tree-row pseudo-class instead.
  • The Bookmarks Toolbar overflow button with anonid chevronPopup is no longer anonymous; it has an ID of "PlacesChevron".
  • The tabs element now has a tabbox property, replacing the old _tabbox property, which has been deprecated (and was never documented).
  • XUL window elements now have the drawintitlebar attribute; if this is true, the window's content area includes the title bar, allowing drawing into the title bar.
  • New TabPinned and TabUnpinned events are available, allowing you to detect when tabs are pinned and unpinned.
  • The new TabAttrModified event is sent when a tab's label, crop, busy, image, or selected attributes change.
  • tab elements now have a pinned attribute, letting you determine whether or not a tab is currently pinned.
  • The setDirectionIndicator class on tree elements hasn't done anything for some time now; now it's not used at all anymore.
  • The window element now has a chromemargin attribute that lets you set the margin between chrome and content on each side of a window; you can use this to draw into the title bar, for example.
  • The window element now has a disablechrome attribute; this is used to hide most of the chrome in a window when it's being used to display in-browser UI, such as about:addons.
  • The window element now has a disablefastfind attribute, which lets you disable the find bar in a window when the content doesn't support it. This is used, for example, by the add-ons panel.
  • Toolbars can now be external to toolboxes, while still being considered a member of the toolbox, by setting the toolboxid property of the toolbar. Also, the toolbox element now has a externalToolbars property, which lists all the toolbars that are considered members of the toolbox.
  • Support has been added for logging XUL templates for debugging purposes.

UI changes affecting developers

The add-on bar
The status bar has been removed in favor of the new add-on bar. You'll need to update your extension to use this if you've been adding UI to the status bar in the past.
Hiding browser chrome
You can now hide the browser's chrome when it's desirable to do so; for example, about:addons does this.


Miscellaneous storage API changes


In addition to the specific changes referenced below, it's important to note that there are no longer any frozen interfaces. All interfaces are now unfrozen, regardless of what the documentation may say. We'll update the documentation over time.

XPCOM changes in Gecko 2.0
Details about changes to XPCOM that impact compatibility in Firefox 4.
This new method returns the global object with which an object is associated; this replaces a common use case of the now-removed __parent__.


Interface changes

Memory management

Infallible memory allocation
Mozilla now provides infallible memory allocators that are guaranteed not to return null. You should read this article to learn how they work and how to explicitly request fallible versus infallible memory allocation.

Other changes

  • Most of the resources contained within Firefox have been combined into a single JAR archive, omni.jar, which improves startup performance by reducing I/O. For details, read About omni.jar.
  • The accessibility.disablecache preference is no longer supported; it was only exposed for debugging purposes and is no longer used.
  • Addons whose GUID changes from one version to another can now be updated properly.
  • As a side effect of the removal of platform-specific directories in add-on bundles, you can no longer provide different default preferences for each platform.
  • By default, extensions are no longer unpacked when they are installed, but are instead run directly from the XPI file. Extensions can use the unpack property in the install manifest to choose the old behavior. Extensions that use binary components, DLLs loaded using js-ctypes, search plugins, dictionaries, and window icons must specify that they need to be unpacked. Extensions that create SQLite database, or do copy things from the filesystem relatively to the extension's directory, may also need to change their code.
  • You may now include extensions that automatically get installed at application startup within a customized Firefox.

Other changes

Only the root chrome.manifest file is loaded
Only the root chrome.manifest file is loaded now; if you need secondary manifest files to be loaded, you can use the manifest command in your root chrome.manifest to load them.
Gopher support removed
The Gopher protocol is no longer supported natively. Continued support is available via the OverbiteFF extension.
Content process event handling
In order to support out-of-process plugins and other multiple-process features, a new API has been introduced to support sending messages across processes.
Bootstrapped extensions
You can now create extensions that can be installed, uninstalled, and upgraded (or downgraded) without requiring a browser restart.
Default plugin removed
The default plugin has been removed. The application plugins folder has also been removed by default, however support for installing plugins via this folder still exists. See bug 533891.
Extension Manager replaced by Addon Manager
nsIExtensionManager has been replaced by AddonManager.
Child HWNDs no longer used
Firefox no longer creates child HWNDs for its internal use on Windows. If you've written an extension that uses native code to manipulate these HWNDs, your extension will not work on Firefox 4. You'll need to either stop using HWNDs or wrap your code that relies on HWNDs in an NPAPI plugin. That's a lot of work, so if you can avoid using HWNDs directly, you should.
Gesture changes
The three finger up and down swipe gestures on trackpads have been changed to, by default, open and close Firefox Panorama view (neé TabCandy). To change these back to the previous scroll-to-top and scroll-to-bottom commands, open about:config and set browser.gesture.swipe.down to cmd_scrollBottom and browser.gesture.swipe.up to cmd_scrollTop.

See also