Firefox 26 for developers

Firefox 26 was released on December 10, 2013. This article lists key changes that are useful not only for web developers, but also Firefox and Gecko developers as well as add-on developers.

Changes for Web developers

CSS

The text-decoration-line property, still prefixed, now considers 'blink' as a valid value, though it doesn't blink the content at all (bug 812995).
The non-standard -moz-text-blink property has been removed (bug 812995).
Support for the image-orientation property, in its CSS Images & Values Level 4 version, that is with the from-image keyword and EXIF support, has been added (bug 825771).
Experimental support for position: sticky has been implemented and can be enabled by pref layout.css.sticky.enabled (bug 886646).
The text-align property now applies to the ::-moz-placeholder pseudo-element (bug 915551).

HTML

The HTMLSelectElement.selectedOptions property has been implemented (bug 596681).
In the <input> element of type email, values with domain labels longer than 63 chars are no more considered valid (bug 884332).
The HTMLInputElement.width and height properties now return 0 when the type is not image (bug 905240).
A <fieldset> element is now invalid, and can be styled using the :invalid pseudo-class, when one of the elements it contains is invalid (bug 717181).

JavaScript

EcmaScript 6 (Harmony) implementation continues!

The ECMAScript 6 compliant syntax for Generators (yield) has been implemented (bug 666399).
Generator/Iterator results are now boxed like { value: foo, done: bool } (bug 907744).
New mathematical methods have been implemented on Math: Math.fround() (bug 900125).
The reserved words cannot be used for function names:such a usage now throws a SyntaxError (bug 907958).
The default parameter syntax has been updated to allow parameters without defaults after default parameters, such as function f(x=1, y). See bug 777060.
GeneratorFunction is implemented (bug 904701).

Interfaces/APIs/DOM

Make the last argument (doctype) to DOMImplementation.createDocument optional (bug 909859).
Implement the new element.classList specification which permits adding/removing several classes with one call (bug 814014).
The URL() constructor have been implemented on the URL interface (bug 887364).
The properties URLUtils.origin, URLUtils.password, and URLUtils.username are now available to all interfaces implementing URLUtils: URL, Location, HTMLAnchorElement, and HTMLAreaElement (bug 887364).
The URL interface is now accessible from Web Workers (bug 887364).
IndexedDB can now be used as a "optimistic" storage area so it doesn't require any prompts and data is stored in a pool with LRU eviction policy, in short temporary storage (bug 785884).
Support for WaveShaperNode.oversample has been added (bug 875277).
Path of the persistent storage has been changed from <profile>/indexedDB to <profile>/storage/persistent (on b2g from /data/local/indexedDB to /data/local/storage/persistent).
The Screen.orientation property and Screen.lockOrientation() method now support the default value, mapping to portrait-primary or landscape-primary, depending of the device (bug 908058) This works only for Firefox OS and Firefox for Android. Firefox Desktop is not supported.
Event constructors can be used in Web workers (bug 910910).
Trying to set the Document.domain property on a page embedded in an <iframe> with the sandbox attribute now throws a security error (bug 907892).
The MessageEvent interface has been updated to comply with the latest spec. The initMessageEvent method has been removed while the interface has now a constructor (bug 848294).
The HTML5 MessageChannel API has been implemented, behind the dom.messageChannel.enabled preference (bug 677638).
Support for VTTCue, behind the media.webvtt.enabled preference, like for all WebVTT-related implementations, has been added (bug 868509).

MathML

Inconsistent renderings of <mmultiscripts>, <msub>, <msup> and <msubsup> have been unified and the error handling of these elements has been improved (bug 827713).

SVG

The inclusion of SVG glyphs inside OpenType, SVG-in-OpenType, has been updated to match the current version of the specification (bug 906521).
The SVGElement.ownerSVGElement() method doesn't throw anymore (bug 835048).

Development tools

The Inspector is now remotable (bug 805526).
The web console text can be selected, ::before and ::after now inspectable, debugger and responsive design features are planned for this release. (https://hacks.mozilla.org/2013/09/new-features-in-the-firefox-developer-tools-episode-26/)