Firefox 50 for developers

To test the latest developer features of Firefox,
install Firefox Developer EditionFirefox 50 was released on November 15, 2016. 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

The default style of <bdo> now sets unicode-bidi with the isolate-override value (bug 1249497).
Setting the <track> element's src attribute now works correctly (bug 1281418).
The referrerpolicy attribute on <area>, <a>, <img>, <iframe> and <link> elements is now available by default (bug 1223838, bug 1264165).

CSS

Border-radiused corners with dashed and dotted styles are now rendered with the specified style instead of a solid style (bug 382721).
The non-standard :-moz-full-screen-ancestor pseudo-class selector has been removed (bug 1199529).
The box-sizing: padding-box has been removed, since it’s no longer a part of the spec and Firefox was the only major browser implementing it (bug 1166728).
The three values isolate, isolate-override, and plaintext of the unicode-bidi property have been unprefixed (bug 1141895).
In quirks mode, the bullet of a list item now inherits the size of the list, like in standards mode (bug 648331).
The :in-range and :out-of-range pseudo-classes have changed behavior to not match disabled or read-only inputs (bug 1264157).
The :any-link pseudo-class is now unprefixed (bug 843579).
The space value for border-image-repeat has been implemented (bug 720531).

JavaScript

The ES2015 Symbol.hasInstance property has been implemented (bug 1054906).
The ES2017 Object.getOwnPropertyDescriptors() method has been implemented (bug 1245024).
The behavior of \W in RegExp with unicode and ignoreCase flags is changed to match recent draft spec. Now it doesn't match to K, S, k, s, and KELVIN SIGN (U+212A), and LATIN SMALL LETTER LONG S (U+017F) (bug 1281739).

Developer Tools

All devtools bugs fixed between Firefox 49 and Firefox 50.

HTTP

The experimental (and deprecated) SPDY 3.1 is now disabled by default bug 1287132.
Support for X-Content-Type-Options has been added (bug 471020).
The cookie prefixes __Host- and __Secure- have been implemented. See Set-Cookie and bug 1283368.
The Referrer-Policy header has been implemented bug 1264164.

Security

The ping attribute of <a> element now abides by the connect-src CSP 1.1 policy directive (bug 1100181).
Support for the sandbox CSP directive has been added (bug 671389).
It's now possible to set a content security policy for workers (bug 959388).
The Navigator.sendBeacon() method no longer throws an exception if the beacon data couldn't be sent due to a Content Security Policy restriction; instead, it returns false as expected (bug 1234813).
Support for RC4 encryption was deprecated in Firefox 36 and disabled by default in Firefox 44. The one-year grace period has ended, so Firefox 50 removes all support for RC4 (Google Chrome removed support for RC4 in August 2016). From now on, any time Firefox encounters RC4 encryption, it will report an SSL_ERROR_NO_CYPHER_OVERLAP error.

Networking

When a error has happened during an asynchronous XMLHttpRequest, the XMLHttpRequest.getAllResponseHeaders() method now returns an empty string (bug 1286744).
Instead of returning a NetworkError, asynchronous XMLHttpRequest that fails for CORS or other network constraints now raises an error that can be catched like any other error (bug 709991).
XMLHttpRequest.getResponseHeader() and XMLHttpRequest.getAllResponseHeaders() now also return empty headers by default. This can be controlled via the preference network.http.keep_empty_response_headers_as_empty_string (bug 918721).
The only-if-cached option has been added to Request.cache (bug 1272436).

DOM

The once option for EventTarget.addEventListener() is now supported (bug 1287706).
The interface NodeList are now iterable and the methods forEach(), values(), NodeList.entries() and NodeList.keys() are now available (bug 1290636).
The interface DOMTokenList are now iterable and the methods forEach(), values(), DOMTokenList.entries() and DOMTokenList.keys() are now available (bug 1290636).
The methods Document.createElement() and Document.createElementNS() now have an optional options parameter for creating custom elements (bug 1276579).

SVG

The allowReorder attribute has been dropped and the behavior it was setting is now the default for SVG <switch> elements (bug 1279690).
The defer keyword for the preserveAspectRatio attribute on SVG <image> elements has been removed to follow the latest SVG2 specification (bug 1280425).

Drag and Drop API

The DataTransfer.items property has been implemented, allowing access to multiple items being dragged and dropped using the HTML Drag and Drop API. To allow this, the DataTransferItem and DataTransferItemList interfaces are now supported as well (bug 906420). This is enabled by default.
The old, obsolete Firefox specific drag and drop API events dragdrop and draggesture are no longer supported. Be sure to update any code still using them to use the HTML drag and drop API (bug 1162050.

Pointer Lock API

The Pointer Lock API is now unprefixed (bug 991899).
Before Firefox 50, requestPointerLock() asked for permission using a doorhanger, and pointer lock would not be enabled until the user granted permission. From Firefox 50, pointer lock is like the fullscreen API: it's granted immediately, but a notification is displayed explaining to the user how to exit (bug 1273351).

IndexedDB

A close event is now sent to the IDBDatabase object when the corresponding database is unexpectedly closed (bug 1151017).

Service Workers

The WindowClient.navigate() method has been implemented. This method lets you open a specified URL into a client window which is being controlled by the service worker (bug 1218148).

WebGL

The EXT_shader_texture_lod WebGL extension has been implemented (bug 1111689).
The texImage methods have been updated for WebGL 2 to implement PBOs (PIXEL_UNPACK_BUFFER) (bug 1280499).

WebRTC

Adding a track to a MediaStream now generates the addtrack event as described in the specification. The event is of type MediaStreamTrackEvent and is fired on the stream to which the track was added. You can use either MediaStream.addEventListener('addtrack', ...) or the MediaStream.onaddtrack property to handle "addtrack" events.
The MediaStreamTrack interface now supports the ended event and the MediaStreamTrack.onended event handler.
Firefox now supports the MediaStreamTrack.readyState property, which indicates whether the track is live or permanently ended.
The MediaStreamTrack methods getConstraints() and getSettings() have been implemented; these let you get the most recently applied set of customized property constraints and the actual values of all of the track's constrainable properties, respectively. The accompanying data types have been documented as well.
The RTCDataChannel.stream property has been removed. This was replaced with RTCDataChannel.id in Firefox 24, but was supported for backward compatibility. Please be sure to update your code to use the id property if you haven't done so yet.

Web Audio API

The PannerNode interface now supports the 3D Cartesian space properties for the position (PannerNode.positionX, PannerNode.positionY, and PannerNode.positionZ) and directionality (PannerNode.orientationX, PannerNode.orientationY, PannerNode.orientationZ) of an audio source.
The interface IIRFilterNode, which implements a general infinite impulse response (IIR) filter, has been implemented.
Throttling in background tabs of timers created by Window.setInterval() and Window.setTimeout() no longer occurs if a Web Audio API AudioContext is actively playing sound. This should help prevent issues with timing-sensitive audio playback (such as music players generating individual notes using timers) in the background (bug 1181073).

Audio/Video

The AlignSetting enum (representing possible values for VTTCue.align) incorrectly previously included the value "middle" instead of "center". This has been corrected (bug 1276130).
The non-standard and experimental method HTMLMediaElement.seekToNextFrame() now seeks to the next frame in the media asynchronously, rather than synchronously, and returns a Promise which resolves once the seek is complete.
The implementation of HTMLTrackElement has been corrected to allow <track> elements to load resources even if not in a document (bug 871747).

Battery API

The navigator.battery property, which has been deprecated since Firefox 43, is now obsolete and has been removed. Use the navigator.getBattery() method instead to get a battery Promise, which will resolve when the BatteryManager is available for use; the BatteryManager is passed into the fulfillment handler for the promise (bug 12593355).

Files and directories

A subset of the File and Directory Entries API has been implemented, to improve compatibility with sites that were previously only compatible with Google Chrome (bug 1265767).
- The asynchronous API interfaces have been implemented, with the caveat that only reading of files is supported; for example, the FileSystemFileEntry.createWriter() method is a no-op.
- These interfaces have been implemented:
  - FileSystem
  - FileSystemEntry (properties only; the methods have not been implemented)
  - FileSystemFileEntry (except for createWriter())
  - FileSystemDirectoryEntry (except for removeRecursively())
  - FileSystemDirectoryReader
- HTMLInputElement.webkitdirectory as well as the webkitdirectory attribute of the <input> element have been implemented; this lets you configure a file input to accept directories instead of files (bug 1258489).
- HTMLInputElement.webkitEntries has been implemented; this returns an array of FileSystemEntry-based objects representing the selected items.
- File.webkitRelativePath has been implemented; this contains the path of the file relative to the root of the containing FileSystemDirectoryEntry that was among the items in the list returned by HTMLInputElement.webkitGetEntries().
- See File and Directory Entries API support in Firefox for details about what we do and do not support in this API.
- These APIs are now enabled by default; some were previously available but only behind a preference (bug 1288683).
We've implemented DataTransferItem.webkitGetAsEntry() as part of the File and Directory Entries API; this lets you obtain a FileSystemEntry representing a dropped file (bug 1289255). This is enabled by default.
The HTMLInputElement.directory property, part of the Directory Upload API proposal, has been renamed to allowdirs (bug 1288681). This property is hidden behind a preference.