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


  • <hr> elements can now be used as separators in <menu> elements (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).



Developer Tools


IndexedDB v2



Web Workers




  • 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.
  • 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.


  • Added 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.
  • Added support for FLAC in MP4 (both with and without MSE) (bug 1303888).
  • 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.




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:
    • 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 (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 (bug 1279635).

See also

Older versions

Document Tags and Contributors

 Last updated by: yisibl,