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This article provides information about the changes in Firefox 55 that will affect developers.

Firefox 55 is the current Beta version of Firefox, and will ship on August 8, 2017.

Changes for Web developers

Developer Tools






New APIs

  • The Collaborative Scheduling of Background Tasks API (also known simply as the Background Tasks API or the requestIdleCallback API) is now enabled by default after being available behind a preference since Firefox 53. This API lets you schedule tasks to be executed when the browser determines that there's free time available before the next repaint occurs, so that your code can make use of that time without causing visible performance lag (bug 1314959).
  • The WebVR 1.1 API is now turned on by default on Windows (and is available on macOS in Nightly). This API exposes virtual reality devices — for example head-mounted displays like the Oculus Rift or HTC Vive — to web apps, enabling developers to translate position and movement information from the display into movement around a 3D scene, and present content into such displays.


DOM events

Selection API


Service Workers/Push


  • When called with a set of constraints that resulted in there being no media sources available, getUserMedia() was incorrectly returning NotSupportedError instead of TypeError. This has been fixed (bug 1349480).




  • Flash content is now "click-to-activate" (bug 1317856). This was immediately put into effect for all users of Nightly, and 50% of beta users. For Firefox 55 release version, the plan is to activate this for 5% of users 2 weeks after release, 25% of users 4 weeks after release, and 100% of users 6 weeks after release (bug 1365714).
  • Flash and other plugins can no longer be loaded from any URL scheme except for http:// and https:// (bug 1335475).

Removals from the web platform




  • The UIEvent.isChar property has never been supported by any browser but Firefox, and it has never been fully-implemented except on macOS. For that reason, it was removed in Firefox 55 to align with other browsers.
  • The proprietary Firefox OS Device Storage API has been removed from the platform (bug 1299500).
  • The aShowDialog parameter of the non-standard Window.find() method (which could be specified to open up a "Find" dialog in the browser) has been removed (bug 1348409).


  • The SVGZoomEvent and SVGZoomEvents interfaces have been removed from the SVG2 spec and Gecko, along with the onzoom <svg> attribute (bug 1314388).

Changes for add-on and Mozilla developers



See also

Older versions

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, dSmurf, wbamberg, fscholz, Ehsan, yisibl, Sebastianz, Sheppy, Zefling
 Last updated by: chrisdavidmills,