Firefox 51, based on Gecko 51, will ship in January 2017. This article provides information about the changes in this release that will affect developers. Nightly builds of what will become Firefox 51 are currently available on the Developer Edition channel. Some experimental features whose release dates are uncertain or undecided are discussed on the page Experimental features in Firefox. Some features described there may be available in Firefox 51.
Changes for Web developers
<hr>elements can now be used as separators in
<menu>elements (bug 870388.
selectionEndattributes now correctly return the current position of the text input cursor when there's no selection, instead of returning 0 (bug 1287655).
:indeterminatefor <input type="radio"> (bug 885359).
<input type="text">(bug 1069015).
::placeholderpseudo-element is now unprefixed (bug 1069012).
- Fixed the
:validCSS pseudo-class which didn't match valid
<form>elements (bug 1285425).
unicode-bidinow also works with vertical writing modes (bug 1302734).
clip-pathare now properly supported; previously, they were aliases of
- Clamp flex line's height (clamping stretched flex items), in single-line auto-height flex container w/ max-height (spec change) (bug 1000957).
- The ES2015
Symbol.toStringTagproperty has been implemented (bug 1114580).
- The ES2015
TypedArray.prototype.toLocaleString()methods have been implemented (bug 1121938).
DateTimeFormat.prototype.formatToParts()method is now available (bug 1289340).
letare now fully ES2015-compliant (bug 950547).
for...ofloops now has a fresh binding for each iteration and no longer throws a
- The deprecated for each...in loop now presents a warning in the console (bug 1293205). Please migrate your code to use the standardized
- Generator functions can't have a label anymore and "
let" as a label name is disallowed now (bug 1288459).
- Deprecated legacy generator functions will now throw when used in method definitions (bug 1199296).
next()method of the iterator protocol will now throw a
TypeErrorif the returned value is not an object (bug 1016936).
- Network Monitor now shows a "Blocked" state for network requests.
- All devtools bugs fixed between Firefox 50 and Firefox 51.
- WebGL2 has been activated by default (and got minor fixes to match the spec).
- IndexedDB version 2 implementation is now complete:
- The non-standard
CanvasRenderingContext2D.mozFillRule() method has been removed; the fill rule can be defined using a parameter of the standard
CanvasRenderingContext2D.fill()method (bug 826619).
CanvasRenderingContext2D.imageSmoothingEnabledhas been unprefixed (bug 768072)
tabindexattribute (bug 778654).
hrefattribute, which renders
xlink:hrefobsolete (bug 1245751).
- You can now use custom data attributes on SVG elements through the
SVGElement.datasetproperty and the
data-*set of SVG attributes (bug 921834).
- CSS Animations used in an SVG image which is presented in an
<img>element now work again; this was an old regression (bug 1190881).
- The non-standard and obsolete
WorkerGlobalScope.oncloseevent handler and
Workeruse of the
closeevent have been removed from Firefox.
- Scripts served with an
text/csvMIME type are now blocked and are not loaded or executed. This happen when they are declared using
<script>, or loaded via
SharedWorker()(bug 1229267 and bug 1288361).
XMLHttpRequest.responseXMLproperty no longer returns a partial
Documentwith a <parsererror> node placed at the top when a parse error occurs attempting to interpret the received data. Instead, it correctly returns
- To match the latest specification an
Acceptheader set with
setRequestHeader()is now sent with such a header, with its value set to
XMLHttpRequest.open()so that, when omitted, the
passwordparameters now default to
null, per the specification (bug 933759).
RTCPeerConnection.removeStream()method has been removed. It was deprecated back in Firefox 22, and has been throwing a
NotSupportedErrorfor a long time. You need to use
RTCPeerConnection.removeTrack()instead, for each track on the stream.
- WebRTC now supports the VP9 codec by default. When added in Firefox 46, VP9 was disabled by default, but when enabled was the preferred codec; however, it has been moved to be the second choice (after VP8) due to its current level of CPU usage.
- The method
HTMLMediaElement.captureStream(), which returns a
MediaStreamcontaining the content of the specified
<audio>. It's worth noting that this is prefixed still as
mozCaptureStream(), and that it doesn't yet exactly match the spec.
- Added FLAC support (FLAC codec) in both FLAC and Ogg containers (bug 1195723). Supported FLAC MIME types are:
audio/x-flac. For FLAC in Ogg, supported MIME types are:
audio/ogg; codecs=flac, and
- Added support for FLAC in MP4 (both with and without MSE) (bug 1303888).
- Throttling in background tabs of timers created by
Window.setTimeout()was changed in Firefox 50 to no longer occur if a Web Audio API
AudioContextis actively playing sound. However, this didn't resolve all scenarios in which timing-sensitive audio playback (such as music players generating individual notes using timers) could fail to work properly. For that reason, Firefox 51 no longer throttles background tabs which have an
AudioContext, even if it's not currently playing sound.
truein all cases (bug 984778).
selectionEndnow correctly return the current position of the text input cursor when there's no selection, instead of returning 0 (bug 1287655).
HTMLImageElementinterface and the corresponding
<img>element now support the
onerrorevent handler, sending
errorevents to the element whenever errors occur attempting to load or interpret images.
- You can now change a Web
Animation's effect by setting the value of its
effectproperty. Previously, this property was read-only (bug 1049975).
- The Permissions API method
Permissions.revoke()has been put behind a preference (
dom.permissions.revoke.enable) and disabled by default since its design and even its very existence is under discussion in the Web Application Security Working Group.
- The Storage API's
StorageManager.estimate()method have been implemented along with the needed supporting code. Storage unit persistence features are not yet implemented. See bug 1267941.
- For privacy reasons, both
BatteryManager.dischargingTimenow round the returned value to the closest 15 minutes (bug 1292655).
- Firefox now supports the
onanimationstartevent handlers, in addition to supporting the corresponding events using
- Firefox now supports the
ontransitionendevent handler (bug 911987).
- The non-standard Simple Push API, mainly intended for use with Firefox OS and now superceded by the W3C Push API, has been completely removed from Gecko (bug 1296579).
- The non-standard Alarms API, mainly intended for use with Firefox OS, has been completely removed from Gecko (bug 1300884).
- Support for prefixes in the Page Visibility API has been removed (bug 812701).
Changes for add-on and Mozilla developers
- New APIs:
- You can now embed a WebExtension in a legacy add-on type (bug 1252215).
- Clipboard access is now supported (bug 1197451)
- The arguments passed to the callback of
tabs.executeScript()have been fixed (bug 1290157)
- localStorage is now cleared when a WebExtension is uninstalled (bug 1213990)
- A changed
Content-Typeheader in Web Extensions is now taken into account (bug 1304331)
- The Mozilla-specific Social API has been substantially changed (largely to remove APIs no longer used), as follows:
MozSocialinterface and the
navigator.mozSocialproperty which supports it have been removed.
- The Social Bookmarks API has been removed.
- The Social chat functionality has been removed.
- The Social Status API has been removed.
- All of the social widgets, except for the Share panel, have been removed. This includes the social sidebar, flyover panels, and so forth.
- All supporting user interface features and functionality for the removed APIs have been removed as well.
- Social service provider manifest properties supporting the removed functionality are no longer supported.
- If an add-on uses
mimeTypes.rdfto provide a file extension to MIME type mapping, it must now register an entry in the
"ext-to-type-mapping"category (bug 306471).
- The Browser API now includes a
detailobject on the event object of the
mozbrowserlocationchangeevent that contains
canGoBackproperties, allowing retrieval of the mozBrowser's back/forward status synchronously (bug 1279635).
- Firefox 50 for developers
- Firefox 49 for developers
- Firefox 48 for developers
- Firefox 47 for developers
- Firefox 46 for developers
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