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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.

Items listed here are tentatively slated for Firefox 51; however, they may be held for a future release if testing shows they're not ready by the time Firefox 51 is due to ship. Please keep an eye on this page to stay up to date on the plans for Firefox 51.

Changes for Web developers

Developer Tools

No change.


No change.







No change.


No change.

Web Workers

Service Workers

No change.


  • 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.



No change.



  • 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.


No change.


No change.


No change.

Changes for add-on and Mozilla developers


No change.


No change.

JavaScript code modules

No change.


No change.


  • 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

Older versions

Document Tags and Contributors

 Contributors to this page: Sheppy, teoli, fscholz, yisibl, isonic, Sebastianz, evilpie, jyavenard
 Last updated by: Sheppy,