Firefox 19 for developers

Firefox 19 was released on February 19, 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

JavaScript

Map and Set objects have changed from having a size() method to a size property (bug 807001)
Map and Set objects also have a clear() method now. (bug 805003)

CSS

Support for the viewport-relative <length> units, vh, vw, vmin, and vmax, has landed (bug 503720)
CSS Flexbox has been unprefixed, but remains disabled by default (bug 801098).
The -moz-initial value has been unprefixed (bug 806068). -moz-initial will be kept for a while as an alias; however, authors are strongly encouraged to switch over to initial.
The CSS text-transform property now supports the full-width keyword, which allows a more seamless inclusion of Latin characters in text using ideographic fixed-width characters, like Chinese or Japanese (bug 774560).
The CSS page-break-inside has been implemented (bug 685012).
The CSS calc() function can now be used on <color-stop> (on <gradient>).
The CSS @page at-rule is now supported (bug 115199). Note that the pseudo-classes :first, :right, and :left are not yet implemented.
The :-moz-placeholder pseudo-class is replaced by the ::-moz-placeholder pseudo-element (bug 737786).
Declarations qualified with !important appearing in @keyframes are now ignored, per spec (bug 784466).

DOM/APIs

The Element.getElementsByTagName, Element.getElementsByTagNameNS and Element.getElementsByClassName methods now return a live HTMLCollection (bug 799464).
The File mozLastModifiedDate property has been implemented. (bug 793955)
The File lastModifiedDate property returns the current date, when the date of the last modification is unknown. (bug 793459
The CanvasRenderingContext2D isPointInStroke method has been implemented (bug 803124).
The HTMLCanvasElement toBlob method has been implemented (bug 648610).
The Node.isSupported and the document.implementation.hasFeature() methods have been changed to always return true (bug 801425).
When calling document.createElement(null), null will now be stringified and works like document.createElement("null").
The TextDecoder and TextEncoder interfaces have been updated to match the latest spec (bug 801487).

XForms

Support for XForms has been removed in Firefox 19.

Changes for add-on and Mozilla developers

Note: A key change in Firefox 19 is that nsresult is now strongly typed. This will help make it easier to detect bugs that are caused by mishandling of return values, but may cause existing code to break if it's making incorrect assumptions in this regard.

getBrowserSelection() now returns the selected text in a text input field. As a result, gContextMenu.isTextSelected will be true when the user selects text in a text input field that is not a password field. (bug 565717)
Dict.jsm: Dict() now takes a JSON String. Dict.toJSON() was added, and it returns a JSON String. (bug 727967)

Interface changes

nsIImgLoadingContent: The parameter (aObserver) of addObserver() method changes from imgIDecoderObserver to imgINotificationObserver. The notify() method of imgINotificationObserver is not scriptable, so you need to use createScriptedObserver() from imgITools.
nsIChannel: The property contentLength changed from long to int64_t.