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
clip-pathbehind the preference
layout.css.clip-path-shapes.enabled, defaulting to
- [css-grid] Implement
fit-content()value for <track-size> (bug 1281320).
- [css-grid] Percentage gutters are wrong calculated on grid containers with indefinite sizes (bug 1279182).
- [css-grid] Reject repeat() column tracks in <grid-template> when there's a grid template area string in the row part (bug 1282643).
calc()with negative percentage can result in negative track size (bug 1282418).
:indeterminatefor <input type="radio"> (bug 885359).
- Implemented the
<input type="text">(bug 1069015).
::placeholderpseudo-element (bug 1069012).
:validCSS pseudo-class doesn't match valid <form>s (bug 1285425).
- Fix initial value for
mask-repeatproperty should be
no-repeat, but is implemented as
unicode-bidinow also works with vertical writing modes (bug 1302734).
clip-pathare now properly supported; previously, they wear aliases of
- 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).
DOM & HTML DOM
- 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).
- 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.
- The non-standard
CanvasRenderingContext2D.mozFillRule() method has been removed; the fill rule can be define by a parameter of the standard
CanvasRenderingContext2D.fill()method (bug 826619).
CanvasRenderingContext2D.imageSmoothingEnabledhas been unprefixed (bug 768072).
- The non-standard and obsolete
WorkerGlobalScope.oncloseevent handler and
Workeruse of the
closeevent have been removed from Firefox.
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.
- For privacy reasons, both
BatteryManager.dischargingTimenow round the returned value to the closest 15 minutes (bug 1292655).
- Our implementation of Page Visibility API doesn't accept prefixed version anymore (bug 812701).
tabindexattribute (bug 778654).
- CSS Animations used in a SVG used in an
<img>element now works again; it was an old regression (bug 1190881).
- Add 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
Changes for add-on and Mozilla developers
- 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.
- Firefox 50 for developers
- Firefox 49 for developers
- Firefox 48 for developers
- Firefox 47 for developers
- Firefox 46 for developers
- Firefox 45 for developers
- Firefox 44 for developers
- Firefox 43 for developers
- Firefox 42 for developers
- Firefox 41 for developers
- Firefox 40 for developers
- Firefox 39 for developers
- Firefox 38 for developers
- Firefox 37 for developers
- Firefox 36 for developers
- Firefox 35 for developers
- Firefox 34 for developers
- Firefox 33 for developers
- Firefox 32 for developers
- Firefox 31 for developers
- Firefox 30 for developers
- Firefox 29 for developers
- Firefox 28 for developers
- Firefox 27 for developers
- Firefox 26 for developers
- Firefox 25 for developers
- Firefox 24 for developers
- Firefox 23 for developers
- Firefox 22 for developers
- Firefox 21 for developers
- Firefox 20 for developers