Firefox 51 for developers

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

Developer Tools

• Network Monitor now shows a "Blocked" state for network requests.

All devtools bugs fixed between Firefox 50 and Firefox 51.

HTML

• Firefox 51 now allows you to use the <hr> element to insert separators in <menu>s, per the specification (bug 870388.
• The <input> and <textarea> elements' selectionStart and selectionEnd attributes now correctly return the current position of the text input cursor when there's no selection, instead of returning 0 (bug 1287655).

CSS

• Implemented inset() value for clip-path behind the preference layout.css.clip-path-shapes.enabled, defaulting to false (bug 1246762).
• [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).
• [css-grid] calc() with negative percentage can result in negative track size (bug 1282418).
• Implemented :indeterminate for <input type="radio"> (bug 885359).
• Implemented the :placeholder-shown pseudo-class on <input type="text"> (bug 1069015).
• Unprefixed ::placeholder pseudo-element (bug 1069012).
• Fix :valid CSS pseudo-class doesn't match valid <form>s (bug 1285425).
• Fix initial value for mask-repeat property should be no-repeat, but is implemented as repeat (bug 1258623).
• The plaintext value of unicode-bidi now also works with vertical writing modes (bug 1302734).
• The fill-box and stroke-box values of clip-path are now properly supported; previously, they wear aliases of border-box (bug 1289011).

JavaScript

• The ES2015 Symbol.toStringTag property has been implemented (bug 1114580).
• The ES2015 TypedArray.prototype.toString() and TypedArray.prototype.toLocaleString() methods have been implemented (bug 1121938).
• The DateTimeFormat.prototype.formatToParts() method is now available (bug 1289340).
• const and let are now fully ES2015-compliant (bug 950547).
• Using const in for...of loops now has a fresh binding for each iteration and no longer throws a SyntaxError (bug 1101653).
• The deprecated for each...in loop now presents a warning in the console (bug 1293205). Please migrate your code to use the standardized for...of loop.
• 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).
• The next() method of the iterator protocol will now throw a TypeError if the returned value is not an object (bug 1016936).

Interfaces/APIs/DOM

DOM & HTML DOM

• Throttling in background tabs of timers created by Window.setInterval() and Window.setTimeout() was changed in Firefox 50 to no longer occur if a Web Audio API AudioContext is 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.
• The XMLHttpRequest.responseXML property no longer returns a partial Document with a <parsererror> node placed at the top when a parse error occurs attempting to interpret the received data. Instead, it correctly returns null (bug 289714).
• The DOMImplementation.hasFeature() now returns true in all cases (bug 984778).
• The method HTMLMediaElement.captureStream(), which returns a MediaStream containing the content of the specified <video> or <audio>. It's worth noting that this is prefixed still as mozCaptureStream(), and that it doesn't yet exactly match the spec.
• The HTMLInputElement and <htmltextareaelement> properties selectionStart and selectionEnd now correctly return the current position of the text input cursor when there's no selection, instead of returning 0 (bug 1287655).
• The HTMLImageElement interface and the corresponding <img> element now support the onerror event handler, sending error events 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 effect property. 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.

Canvas

• 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).
• The CanvasRenderingContext2D.imageSmoothingEnabled has been unprefixed (bug 768072).

WebGL

No change.

IndexedDB

• Supports for the new IDBObjectStore.getKey() method has been added (bug 1271506).

Web Workers

• The non-standard and obsolete WorkerGlobalScope.onclose event handler and Worker use of the close event have been removed from Firefox.

Service Workers

No change.

WebRTC

• The RTCPeerConnection.removeStream() method has been removed. It was deprecated back in Firefox 22, and has been throwing a NotSupportedError for 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.

New APIs

No change.

Others

• For privacy reasons, both BatteryManager.chargingTime and BatteryManager.dischargingTime now 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).

MathML

No change.

SVG

• Added tabindex attribute (bug 778654).
• Added href attribute, which renders xlink:href obsolete (bug 1245751).
• You can now use custom data attributes on SVG elements through the SVGElement.dataset property 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).

Audio/video

• Add FLAC support (FLAC codec) in both FLAC and Ogg containers (bug 1195723). Supported FLAC MIME types are: audio/flac and audio/x-flac. For FLAC in Ogg, supported MIME types are: audio/ogg; codecs=flac, and video/ogg; codecs=flac.

HTTP

No change.

Networking

No change.

Security

No change.

Changes for add-on and Mozilla developers

Interfaces

No change.

XUL

No change.

JavaScript code modules

No change.

XPCOM

No change.

Other

• The Mozilla-specific Social API has been substantially changed (largely to remove APIs no longer used), as follows:
  • The MozSocial interface and the navigator.mozSocial property 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.

See also

• Site Compatibility for Firefox 51

Older versions