Firefox 55 for developers

Firefox 55, based on Gecko 55, will ship in August 2017. This article provides information about the changes in this release that will affect developers. Nightly builds of what will become Firefox 55 are currently available on the Nightly 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 55.

Items listed here are tentatively slated for Firefox 55; however, they may be held for a future release if testing shows they're not ready by the time Firefox 55 is due to ship. Please keep an eye on this page to stay up to date on the plans for Firefox 55.

Changes for Web developers

Developer Tools





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).

New API: Background Tasks API

  • 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).


Removals from the web platform



  • The UIEvent.isChar property has never been supported by any browser but Firefox, and there it's never been fully-implemented except on macOS. For that reason, it's been 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, fscholz, Sebastianz, yisibl, Sheppy, Zefling
 Last updated by: chrisdavidmills,