To test the latest developer features of Firefox, install Firefox Developer Edition Firefox 42 was released on November 3, 2015. 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

Developer Tools

Highlights:

• Debugging Firefox for Android over Wi-Fi
• Firefox OS Simulator configuration in WebIDE
• CSS filter presets

All devtools bugs fixed between Firefox 41 and Firefox 42.

CSS

• Verticalwriting-mode is now supported with rtl scripts (Firefox bug 1131451).
• The values ofcaption-side are now relative to the table, and changing actual meaning according to itswriting-mode valueFirefox bug 1202993.
• Non-standard properties like-moz-margin-start are now aliases of their standard counterpart (margin-inline-start, …). Previously, it was the opposite. This affects the value returned byCSSStyleDeclaration.cssText and iteration of the properties in a style rule, which both use the canonical form (Firefox bug 1118103).
• The prefixed version of CSS gradients can be turned off by setting thelayout.css.prefixes.gradients preference tofalse.
• Several old bugs withfloat and margin collapsing behavior have been fixed (Firefox bug 478834,Firefox bug 538194, andFirefox bug 451791).

HTML

• Experimental support for thereferrer attribute of the<img> (Firefox bug 1166910),<iframe> (Firefox bug 1175736),<a> and<area> (Firefox bug 1174913) has been added. There is no effect by default, thenetwork.http.enablePerElementReferrer beingfalse by default.

JavaScript

• TheReflect object has been implemented (Firefox bug 987514).
• The implementation of theProxyhandler.ownKeys() trap has been updated to match the final ES2015 specification (Firefox bug 1049662).
• CallingMap,Set, orWeakMap withoutnew, will now throw aTypeError (Firefox bug 1083752).

Interfaces/APIs/DOM

DOM & HTML DOM

• Images with anid are no more listed as property of theWindow interface:<img> isn't accessible aswindow.login anymore. This behavior was introduced in Firefox 26 and has been removed to match a later change in the spec. (Firefox bug 959992)

• MouseEvent.offsetX andMouseEvent.offsetY have been added (Firefox bug 69787).

• TheHTMLInputElement interface has been experimentally extended to handle the upload of directories (Firefox bug 1164310). These four members can be exposed by setting thedom.input.dirpicker preference totrue:

  • HTMLInputElement.directory
  • HTMLInputElement.isFilesAndDirectoriesSupported
  • HTMLInputElement.getFilesAndDirectories()
  • HTMLInputElement.chooseDirectory()

• TheDirectory interface has been experimentally extended (Firefox bug 1177688). The two membersDirectory.path andDirectory.getContents can be exposed by setting thedom.input.dirpicker preference totrue.

• TheHTMLMediaElement.mozSrcObject has been renamed inHTMLMediaElement.srcObject (Firefox bug 1175523).

Service Workers

• Request.context has been removed (Firefox bug 1188062).
• ThePush API has been activated by default on Firefox for Desktop (Firefox bug 1153499), but only on the Nightly/Dev Edition/Beta channels. It is being deactivated by default in the Release channel until consumer UX and debugging features have been properly implemented (Firefox bug 1207875.) You can still activate it by enabling thedom.push.enabled pref inabout:config.
• ThePushManager.hasPermission() method has been deprecated in the spec, and replaced with thePushManager.permissionState() method. Firefox has updated its implementation to reflect this (Firefox bug 1183853.)
• Service-worker-related additions to theNotifications API have been implemented (Firefox bug 1114554), but are disabled in this release.

Web Animations API

Our experimental implementation ofWeb Animations API have been extended to support:

• TheAnimationPlayer.playbackRate property (Firefox bug 1127380).
• TheCSSAnimation andCSSTransition interfaces (Firefox bug 1178186).
• TheAnimation.reverse() method (Firefox bug 1150808).
• TheAnimationPlaybackEvent interface has been added andcancel andfinish are now fired onAnimation (Firefox bug 1178664).

Web Components

Our experimental implementation of theShadow DOM has been modified:

• When trying to invokeNode.cloneNode() on aShadowRoot, throws aDataCloneError exception (Firefox bug 1176757).
• When invokingDocument.importNode() with aShadowRoot as an argument, we now throws aNotSupportedErrorDOMException (Firefox bug 1177914).
• When invokingDocument.adoptNode() with aShadowRoot as an argument, we now throws aHierarchyRequestErrorDOMException (Firefox bug 1177991).

WebGL

• WebGL2WebGLTransformFeedback has been implemented (Firefox bug 1048724).
• To obtain a WebGL2 context,HTMLCanvasElement.getContext() now takeswebgl2 instead ofexperimental-webgl2 (Firefox bug 1187174).

WebRTC

• TheRTCPeerConnection.createOffer()'s options data type,RTCOfferOptions, has been updated to more closely match the specification. Among other changes, it is now based upon theRTCOfferAnswerOptions dictionary, which provides thevoiceActivityDetection option.
• TheRTCAnswerOptions dictionary has been added. This is the type used for theoptions parameter tocreateAnswer().
• TheRTCICECandidatePairStats.mozPriority has been renamed toRTCICECandidatePairStats.priority (Firefox bug 1184426).

New APIs

• TheImageBitmap interface, theWindow.createImageBitmap() andWorkerGlobalScope.createImageBitmap() method have been implemented. They are available on regular window scripts and in Web workers and allow efficient posting of images between window and worker contexts (Firefox bug 1044102).

Miscellaneous

• TheIDBCursorWithValue interface is now available inWeb workers (Firefox bug 1188115).
• Error events sent inWeb workers doesn't bubble anymore (Firefox bug 1188141).
• Media Source Extensions (MSE) has been activated for all websites, and no more for a white list of sites (Firefox bug 1185611).
• The non-standard and deprecatedWindow.mozRequestAnimationFrame() has been removed (Firefox bug 909154). Use the standardWindow.requestAnimationFrame() instead.
• Speech synthesis (text-to-speech) has been implemented in Firefox Desktop for Windows, hidden behind themedia.webspeech.synth.enabled flag inabout:config (Firefox bug 1003457.) SeeWeb Speech API for more information.

MathML

No change.

SVG

No change.

Audio/Video

No change.

HTTP

• Firefox 41 and earlier versions were incorrectly accepting undefined or invalid pseudo-header fields in HTTP/2 responses. This is now fixed and the only pseudo-header field accepted from Firefox 42 is the:status as per the specification. Response headers containing arbitrary fields are considered malformed. (Firefox bug 1136727)

Networking

• The CSPupgrade-insecure-requests directive has been implemented (Firefox bug 1139297).

Security

• EV certificates with a validity greater than 39 months are now considered and handled as DV certificates (Firefox bug 1145679).

Changes for add-on and Mozilla developers

Interfaces

nsIContentPolicy

• TheTYPE_EMBED constant has been added tonsIContentPolicy to allow Gecko internals and add-on code to better differentiate different types of requests. Previously,TYPE_OBJECT was used for these cases (Firefox bug 1148030).
• Similarly, theTYPE_SUBDOCUMENT constants has been split intoTYPE_FRAME andTYPE_IFRAME (Firefox bug 1148044).

XUL

No change.

JavaScript code modules

No change.

XPCOM

No change.

Other

No change.