To test the latest developer features of Firefox,
install Firefox Developer Edition Firefox 43 was released on December 15, 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
- Server logging in the Web Console
- Quickly find the rule that overrode a CSS declaration
- "Use in Console" context menu item in Inspector
- "Strict" option for filtering in the Rules view
- Network entries in the Console now link to the Network Monitor
- Markup view shows indicators for pseudo-classes locked for elements
- Support for the standard, unprefixed version of
hyphenshas been landed (bug 953408).
- The shorthand property
fonthas been updated to accept
font-stretchvalues (bug 1057680).
- To match a latest evolution of the specification, the
:fullscreenpseudo-class now selects the whole stack of elements in full screen, and not only the top-level one (bug 1199522).
- The deprecated SVG values for the
tb-rl, have been added in CSS as aliases to standard properties (bug 1205787).
<img>with ICO image containing multiple frames, the intrinsic dimension of the image is set to the one of the largest frame, and no more of the smallest frame bug 1201796.
- The value of the document's viewport (defined with
- The new ES2016 methods
TypedArray.prototype.includes()are now enabled by default (bug 1070767).
Changes regarding the
- To match the ES2015 specification, arrow functions no longer have their own
argumentsobject is now lexically bound (inherited from the outer function). In most cases, rest parameters are a good alternative
(...args) => args[i], see bug 889158.
- The arguments object is now allowed in conjunction with rest parameters (bug 1133298).
- From now on, a mapped
argumentsobject in non-strict functions is only provided if the function does not contain any rest parameters, any default parameters or any destructured parameters (bug 1175394).
DOM & HTML DOM
- A new feature called locale-aware sorting has been added allowing for the creation of indexes with a locale specified, which can then be used to sort data according to the rules of that locale (bug 871846). This is a non-standard Firefox-specific feature.
- As per the specification, if
ExtendableEvent.waitUntil()is called outside of the
ExtendableEventhandler, Firefox will now throw an
InvalidStateError; in addition, multiple calls to
waitUntil()will now stack up, and the resulting promises will be added to the list of extend lifetime promises (bug 1189644).
PushMessageDatamethods have been implemented (bug 1149195).
HTMLCanvasElement.captureStream()method has been activated by default (bug 1177276).
- The non-standard constraint style option list for
RTCOfferOptionshas been deprecated and will be removed entirely in Firefox 44; see this post on the site compatibility blog for details.
- The Battery Status API now uses the new promise syntax for
Navigator.getBattery(), as specified in the recent evolution of the specificiation (bug 1050749).
User-Agentheader is no longer in the list of forbidden header names so it can now be set in a Fetch
Headersobject, via XHR
XMLHttpRequest.setRequestHeader(),… (bug 1188932).
MediaRecorder.MediaRecorder()constructor can now accept an options dictionary as a parameter, which allows you to set custom bitrates for the audio/video to be recorded (bug 1161276).
PerformanceObserverinterface, belonging to the Performance Timeline API has been implemented (bug 1165796).
- The Frame Timing API has been added: the
PerformanceCompositeTiminginterfaces are now availble (bug 1191178).
- The modern Screen Orientation API has been implemented: unprefixed
ScreenOrientationinterface are now available (bug 1131470). The non-standard
Screen.mozUnlockOrientation()will be removed in the future.
- Under Linux, like under Windows,
Event.timeStampnow returns a
- Experimental support for
selectstart, as well as the
GlobalEventHandlers.onselectstartevent handlers property has been added (bug 571294). The
selectionchangeevent is fired on the
Documentif the associated
Selectionobject is concerned, or on the specific
HTMLTextAreaElement(bug 1196479). This feature is controlled by the
dom.select_events.enabledpreference, that defaults to
false, except on Nightly.
- Support for
MouseEvent.offsetYhave been activated on Firefox for Android and Firefox OS (bug 1204841).
HTMLCanvasElement.mozFetchAsStream()method has been removed (bug 1206030).
- The constructor
Request()as well as
fetch()will now raise a
TypeErrorexception when used with a URL containing a username and password (bug 1195820).
- Access to Web Storage (i.e.
sessionStorage) from third-party IFrames is now denied if the user has disabled third-party cookies (bug 536509).
- This whitelist has even been removed in Nightly and Aurora/Dev Edition of the browser (bug 1201023). It is currently scheduled that this removal will also happen for Beta and Release versions for the next version (Firefox 44).
- Subresource integrity has been implemented for
<link>that links to stylesheets (bug 992096).
Changes for add-on and Mozilla developers
- In preparation for future releases to switch over to multi-process content, NPAPI plugins can no longer be run in the same process as the page content. The preferences starting with
dom.ipc.pluginsare no longer used.
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