Firefox 24 for developers

Changes for Web developers

The two values -moz-zoom-in and -moz-zoom-out of the cursor property have been unprefixed to zoom-in and zoom-out (Firefox bug 772153).
To match the specification, the keywords not, only, and, and or cannot be used as media types anymore (Firefox bug 757554).

The <track> element has been implemented behind the media.webvtt.enabled preference, and is disabled by default. (Firefox bug 833385).

Arrow functions are no longer automatically in strict mode unless explicitly requested with "use strict" (Firefox bug 852762).
The String.prototype.repeat JS method has been implemented (Firefox bug 815431).
The Set.prototype.values(), Set.prototype.keys() and Set.prototype.entries() methods on Set objects have been implemented (Firefox bug 869996).

Support for the Range() constructor has been added (Firefox bug 868999).
Support for the Text() constructor has been added (Firefox bug 869000).
Support for the Comment() constructor has been added (Firefox bug 869006).
Support for the DocumentFragment() constructor has been added (Firefox bug 869002).
The FocusEvent interface has been implemented (Firefox bug 855741).
Support for the ChildNode.remove() method has been added (Firefox bug 856629).
The WebVTT interfaces related to the <track> element, HTMLTrackElement, TextTrack, TextTrackCue, TextTrackList, and TextTrackCueList have been implemented behind the media.webvtt.enabled property, defaulting to false (Firefox bug 833385).
The Gamepad interface, and Navigator.getGamepads have been implemented behind the dom.gamepad.enabled property, defaulting to false (Firefox bug 690935).
On desktop Firefox only, HTMLCanvasElement.getContext() can now take the webgl value, in addition to experimental-webgl (Firefox bug 870232).
The non-standard method mozLoadFrom() of HTMLMediaElement has been removed (Firefox bug 877135).

The Network inspector now lets you filter by content type (CSS/image/font etc.) and see the relevant size and load times.
The DevTools options panel to the left lets you disable/enable JavaScript temporarily.
Extension developers may use the new Browser Console for Chrome-level scripts (Replaces Error Console).
The source map syntax has been changed to use //# instead of //@ (Firefox bug 870361).

The dir attribute for controlling directionality of formulas on e.g. <math> or <mrow> elements is now equivalent to using the direction CSS property.
The equal sign ("=") is now stretchable.
The "updiagonalarrow" value for the notation attribute on <menclose> elements has been added.

Doc Shells have now the allowMedia attribute to disable media playing (Firefox bug 759964).
Sherlock search plugins in the application directory or profile won't be loaded anymore (Firefox bug 862143).