Firefox 20 for developers

Firefox 20 was released on April, 2nd 2013. This article provides information about the changes in this release that will affect developers.

Changes for Web developers

HTML

Support for the download attribute on the <a> and <area> element has been added (Firefox bug 676619).
The value auto for the global attribute dir has been implemented (Firefox bug 548206).
The global attribute contextmenu now works on Firefox for Android (Firefox bug 736321).

JavaScript

Support for the WeakMap.prototype.clear() method, recently added to the Harmony (ECMAScript 2015) draft proposal has been added (Firefox bug 814562).
Support for the Math.imul() method, a C-style 32-bit multiplication function. Though proposed for Harmony (ECMAScript 2015) it has not yet been accepted and still is non-standard (Firefox bug 808148).
Web apps using draggable text with Kinetic 3.x are working, even when using the Cairo canvas backend (Firefox bug 835064).
The for each...in statement has been deprecated and should not be used. Consider using the new for...of statement (Firefox bug 804834).
Support for the Map.prototype.keys(), Map.prototype.values(), and Map.prototype.entries() has been added (Firefox bug 817368).

CSS

CSS Flexbox is now available by default in pre-release builds only (excluding beta versions). It can be enabled in release and beta versions by setting the layout.css.flexbox.enabled about:config preference to true.
The mask-type property from the CSS Masking specification has been added (Firefox bug 793617).
Experimental support for the :scope pseudo-class has been added. Enabled by default in Aurora and Nightly, it can be enabled in release and beta versions by setting the layout.css.scope-pseudo.enabled about:config preference to true (Firefox bug 648722).

DOM/APIs

HTMLMediaElement supports now playbackRate (both read and write), with pitch correction. Pitch correction can be controlled using the property mozPreservesPitch (Firefox bug 495040).
CSSOM: Support for the new CSSGroupingRule and CSSConditionRule has been added (Firefox bug 814907).
CSSOM: On CSSRule the constant CSSRule.MOZ_KEYFRAME_RULE and CSSRule.MOZ_KEYFRAMES_RULE have been unprefixed to CSSRule.KEYFRAME_RULE and CSSRule.KEYFRAMES_RULE. The prefixed version is temporarily kept for helping Web author to transition their code Firefox bug 816431).
CSSOM: It is now possible to set the value of conditionText for CSSMediaRule (Firefox bug 815021).
The DOMParser parseFromStream and parseFromBuffer methods are no longer available from web content (Firefox bug 816410).
The XMLSerializer serializeToStream method is no longer available from web content (Firefox bug 816410).
TextDecoder and TextEncoder interfaces are now available in Workers (Firefox bug 795542).
Support for the CSS.supports() method has been added, behind the layout.css.supports-rule.enabled pref (off by default) (Firefox bug 779917).
Support for UndoManager has been added (Firefox bug 617532).
The CSSOM Document.caretPositionFromPoint() method, which returns a CaretPosition has been implemented.
The index argument of the HTMLTableRowElement.insertCell() and HTMLTableElement.insertRow() methods has been made optional as per HTML specification.
Navigator.getUserMedia, still prefixed as Navigator.mozGetUserMedia is now activated by default.
The third, optional, transfer argument of Window.postMessage is now supported. It allows to transfer a sequence of transferable objects to the destination (Firefox bug 822094).
The non-standard Window.sizeToContent() method now clamps the minimal size: the window cannot be forced anymore to small sizes preventing the user from interacting with (Firefox bug 764240).
Blend modes, such as overlay, color-burn, hue, etc. have been added to the Canvas CanvasRenderingContext2D.globalCompositeOperation property (Firefox bug 748433).
The prefixed version of window.indexedDB — window.mozIndexedDB — was reintroduced to Gecko, so that bad cross-browser prefixing code (such as var indexedDB = window.indexedDB || window.webkitIndexedDB …) doesn't break in Firefox. A better approach is window.indexedDB = window.indexedDB || window.webkitIndexedDB … (see Firefox bug 770844.)

SVG

The implementation of the contentScriptType and contentStyleType properties has been removed from SVGSVGElement along with the removal from SVG2 (Firefox bug 819731).

MathML

To help MathML authors debugging "invalid-markup" errors in their documents, MathML parsing errors (such as having too many / too few child elements) and warnings about deprecated attributes or wrong attribute values are now reported to the error console.
The scriptminsize attribute now accepts unitless values and percent values. They are interpreted as multiples of the default value ("8pt").
Unitless values are now also allowed for the mathsize and fontsize attributes; they multiply the default value.

Changes for add-on and Mozilla developers

ECMAScript for XML (E4X) is now fully disabled for all chrome and content scripts. It was previously disabled for content in Firefox 17 and has been removed entirely for Firefox 21. Use DOMParser/DOMSerializer or a non-native JXON algorithm instead.
The nsIDOMParserJS interface no longer exists (Firefox bug 816410). See nsIDOMParser for alternatives.
Content Preferences: The nsIContentPrefService interface is now deprecated and the asynchronous nsIContentPrefService2 storage API has been implemented.
The nsIProfile and nsIProfileChangeStatus interfaces have been removed, along with other code supporting the pre-Firefox profile management system. You probably weren't using these interfaces, but if you were, you should stop doing so. This prevents defunct parts of the profile management system from vetoing the shutdown process.
The nsIEventSource interface no longer exists (Firefox bug 819639).

Firefox 20 for developers

Changes for Web developers

HTML

JavaScript

CSS

DOM/APIs

SVG

MathML

Changes for add-on and Mozilla developers

See also

Older versions