Firefox 51 for developers

To test the latest developer features of Firefox, install Firefox Developer EditionFirefox 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




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 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.


  • 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 setInterval() 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.





Changes for add-on and Mozilla developers



  • 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).

Older versions