Firefox 51 for developers

To test the latest developer features of Firefox, install Firefox Developer Edition. Firefox 51 was released on January 24, 2017. 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

HTML

<hr> elements can now be used as separators in <menu> elements (Firefox 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 (Firefox bug 1287655).

CSS

Implemented :indeterminate for <input type="radio"> (Firefox bug 885359).
Implemented :placeholder-shown for <input type="text"> (Firefox bug 1069015).
The ::placeholder pseudo-element is now unprefixed (Firefox bug 1069012).
Fixed the :valid CSS pseudo-class which didn't match valid <form> elements (Firefox bug 1285425).
The plaintext value of unicode-bidi now also works with vertical writing modes (Firefox bug 1302734).
The fill-box and stroke-box values of clip-path are now properly supported; previously, they were aliases of border-box (Firefox bug 1289011).
Clamp flex line's height (clamping stretched flex items), in single-line auto-height flex container w/ max-height (spec change) (Firefox bug 1000957).

JavaScript

The ES2015 Symbol.toStringTag property has been implemented (Firefox bug 1114580).
The ES2015 TypedArray.prototype.toString() and TypedArray.prototype.toLocaleString() methods have been implemented (Firefox bug 1121938).
The DateTimeFormat.prototype.formatToParts() method is now available (Firefox bug 1289340).
const and let are now fully ES2015-compliant (Firefox bug 950547).
Using const in for...of loops now has a fresh binding for each iteration and no longer throws a SyntaxError (Firefox bug 1101653).
The deprecated for each...in loop now presents a warning in the console (Firefox 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 (Firefox bug 1288459).
Deprecated legacy generator functions will now throw when used in method definitions (Firefox bug 1199296).
The next() method of the iterator protocol will now throw a TypeError if the returned value is not an object (Firefox bug 1016936).
Child-indexed pseudo-class selectors should match without a parent (Firefox bug 1300374).

Developer Tools

WebGL

WebGL 2 is now enabled by default. See webglsamples.org/WebGL2Samples for a few demos.
- WebGL 2 provides the WebGL2RenderingContext interface that brings OpenGL ES 3.0 to the <canvas> element.
- New features include:
  - 3D textures,
  - Sampler objects,
  - Uniform Buffer objects,
  - Sync objects,
  - Query objects,
  - Transform Feedback objects,
  - Promoted extensions that are now core to WebGL 2: Vertex Array objects, instancing, multiple render targets, fragment depth.
The WEBGL_compressed_texture_es3 extension (implemented in Firefox 46) has been renamed to WEBGL_compressed_texture_etc (Firefox bug 1316778) and is no longer included by default in WebGL 2 contexts (Firefox bug 1306174).
The EXT_disjoint_timer_query extension has been updated to use WebGLQuery objects instead of WebGLTimerQuery objects (Firefox bug 1308057).
The OES_vertex_array_object extension now uses the WebGL 2 WebGLVertexArrayObject object instead of its own WebGLVertexArrayObjectOES object (Firefox bug 1318523).
You can now use ImageBitmap objects as a sources for texture images in methods like WebGLRenderingContext.texImage2D(), WebGLRenderingContext.texSubImage2D(), WebGL2RenderingContext.texImage3D(), or WebGL2RenderingContext.texSubImage3D() (Firefox bug 1324924).

IndexedDB v2

IndexedDB version 2 implementation is now complete:
- Supports for the new IDBObjectStore.getKey() method has been added (Firefox bug 1271506).
- Supports for IDBCursor.continuePrimaryKey() method has been added (Firefox bug 1271505).
- Binary keys are now supported (Firefox bug 1271500).
- See also "What's new in IndexedDB 2.0?" – Mozilla hacks

Canvas

The non-standard CanvasRenderingContext2D.mozFillRule() method has been removed; the fill rule can be defined using a parameter of the standard CanvasRenderingContext2D.fill() method (Firefox bug 826619).
The CanvasRenderingContext2D.imageSmoothingEnabled has been unprefixed (Firefox bug 768072)

SVG

Added tabindex attribute (Firefox bug 778654).
Added href attribute, which renders xlink:href obsolete (Firefox bug 1245751).
You can now use custom data attributes on SVG elements through the HTMLElement.dataset property and the data-* set of SVG attributes (Firefox bug 921834).
CSS Animations used in an SVG image which is presented in an <img> element now work again; this was an old regression (Firefox bug 1190881).

Web Workers

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

Networking

Scripts served with an image/*, video/*, audio/* or text/csv MIME type are now blocked and are not loaded or executed. This happen when they are declared using <script>, or loaded via WorkerGlobalScope.importScripts(), Worker(), SharedWorker() (Firefox bug 1229267 and Firefox bug 1288361).
Support for SHA-1 certificates from publicly-trusted certificate authorities has been removed (Firefox bug 1302140). See also Phasing Out SHA-1 on the Public Web for more information.
New WoSign and StartCom certificates will no longer be accepted (Firefox bug 1309707), see Distrusting New WoSign and StartCom Certificates for more information.
The PAC FindProxyForURL(url, host) function now strips paths and queries from https:// URLs to avoid information leakage (see Firefox bug 1255474 and CVE-2017-5384).

XHR

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 (Firefox bug 289714).
To match the latest specification an XMLHttpRequest without an Accept header set with setRequestHeader() is now sent with such a header, with its value set to */* (Firefox bug 918752).
Fixed XMLHttpRequest.open() so that, when omitted, the username and password parameters now default to null, per the specification (Firefox bug 933759).

WebRTC

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

Audio/video

Added FLAC support (FLAC codec) in both FLAC and Ogg containers (Firefox 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.
Added support for FLAC in MP4 (both with and without MSE) (Firefox bug 1303888).
Throttling in background tabs of timers created by Window.setInterval(), WorkerGlobalScope.setInterval(), setTimeout() and 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.

DOM

The DOMImplementation.hasFeature() now returns true in all cases (Firefox bug 984778).
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 (Firefox 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 (Firefox 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 Navigator.storage property and StorageManager.estimate() method have been implemented along with the needed supporting code. Storage unit persistence features are not yet implemented. See Firefox bug 1267941.
For privacy reasons, both BatteryManager.chargingTime and BatteryManager.dischargingTime now round the returned value to the closest 15 minutes (Firefox bug 1292655).

Events

Firefox now supports the onanimationstart, onanimationiteration, and onanimationend event handlers, in addition to supporting the corresponding events using addEventListener() (Firefox bug 911987).
Firefox now supports the ontransitionend event handler (Firefox bug 911987).

Security

When login pages (i.e., those containing an <input type="password"> field) are created so that they would be submitted insecurely, Firefox displays a crossed-out lock icon in the address bar to warn users (Firefox bug 1319119). See Insecure passwords for more details.

Removals

The non-standard Simple Push API, mainly intended for use with Firefox OS and now superseded by the W3C Push API, has been completely removed from Gecko (Firefox bug 1296579).
The non-standard Alarms API, mainly intended for use with Firefox OS, has been completely removed from Gecko (Firefox bug 1300884).
Support for prefixes in the Page Visibility API has been removed (Firefox bug 812701).

Changes for add-on and Mozilla developers

WebExtensions

New APIs:
You can now embed a WebExtension in a legacy add-on type (Firefox bug 1252215).
Clipboard access is now supported (Firefox bug 1197451)
The arguments passed to the callback of tabs.executeScript() have been fixed (Firefox bug 1290157)
localStorage is now cleared when a WebExtension is uninstalled (Firefox bug 1213990)
A changed Content-Type header in Web Extensions is now taken into account (Firefox bug 1304331)

Other

The multiprocessCompatible property of install.rdf must now be explicitly set to false to prevent multiprocess from being enabled in Firefox when the add-on is installed.
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.
If an add-on uses mimeTypes.rdf to provide a file extension to MIME type mapping, it must now register an entry in the "ext-to-type-mapping" category (Firefox bug 306471).
The Browser API now includes a detail object on the event object of the mozbrowserlocationchange event that contains canGoForward/canGoBack properties, allowing retrieval of the mozBrowser's back/forward status synchronously (Firefox bug 1279635).