Firefox 55 for developers

Firefox 55 was released on August 8, 2017. This article lists key changes that are useful for web developers.

Changes for Web developers

Developer Tools






New APIs

  • The Collaborative Scheduling of Background Tasks API (also known 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 (Firefox 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.
  • The Intersection Observer API — which provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport — has been added (Firefox bug 1321865).


DOM events

Selection API


Service Workers/Push

Web Audio API

  • A non-standard constructor (which accepted a string enum value indicating the purpose for which the context would be used) for AudioContext interface was causing errors to be thrown when the options parameter was provided. We have removed the non-standard constructor. However, please note that the options parameter is not yet supported in Firefox and is currently ignored (Firefox bug 1361475).


  • getUserMedia() now provides a stereo audio stream by default if the source device provides stereo sound; support to specifically request mono input will come in Firefox 56. This only works on desktop at this time; mobile Firefox does not currently support stereo audio input sources (Firefox bug 971528).
  • The getUserMedia() media capabilities, constraints, and settings autoGainControl and noiseSuppression now match the spec; formerly they were moz-prefixed (Firefox bug 1366415).
  • When called with an empty constraints set, getUserMedia() was incorrectly returning NotSupportedError instead of TypeError. This has been fixed (Firefox bug 1349480).
  • The following new WebRTC statistics are available: framesEncoded, pliCount, nackCount, and firCount (Firefox bug 1348657).
  • The RTCInboundRTPStreamStats dictionary field formerly called mozRtt has been renamed to roundTripTime to match the specification; in addition, its behavior has been adjusted to match the standard: it contains a double-precision floating point value which estimates the round-trip time based on the RTCP timestamps in the RTCP Receiver Report, measured in seconds (following the algorithm described in RFC 3550, section 6.4.1). (Firefox bug 1344970). However, please be aware that this property is moving to a different dictionary (RTCRemoteInboundRTPStreamStats) soon (Firefox bug 1380555).
  • The RTCRTPStreamStats dictionary now includes the fields firCount, pliCount, and nackCount. These return low-level information that can be used to determine how reliable the connection is (Firefox bug 1348657).
  • The RTCOutboundRTPStreamStats dictionary now includes the field framesEncoded, which reports the number of frames that have been successfully encoded for the stream; with this information, you can compute the frame rate (Firefox bug 1348657).
  • On Android, there's now a pref to turn on hardware video encoding to improve video call performance and save on battery. To be enabled by default in Firefox 56 (Firefox bug 1265755).

Encrypted Media Extensions API

  • Firefox currently allows Encrypted Media Extensions to be used in insecure contexts, despite this not being allowed in the specification. This will be changed in the near future, and starting in Firefox 55, deprecation warnings are output to the web console when this is done. (Firefox bug 1361000).
  • Firefox currently doesn't require that at least one MediaKeySystemCapabilities object be included in the suggestedConfigurations parameter passed into Navigator.requestMediaKeySystemAccess(), which the specification does mandate. Starting in Firefox 55, a warning is output to the web console when any audio or video configuration is specified without specifying supported codecs. Soon, failure to include a valid configuration for one or more of audio and video will throw an exception Firefox bug 1368683).




  • Flash content is now "click-to-activate" (Firefox 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 (Firefox bug 1365714).
  • Flash and other plugins can no longer be loaded from any URL scheme except for http:// and https:// (Firefox bug 1335475).


  • Firefox on Linux can now be made to run in headless mode using the -headless flag (see Firefox bug 1356681).

Removals from the web platform


  • The xml:base attribute can no longer be used to set the base URL for paths appearing in the style attribute, for example — <div xml:base="" style="background:url(picture.jpg)"></div> (Firefox bug 1350521).
  • The <style> element's scoped attribute has been hidden behind a pref (layout.css.scoped-style.enabled) in content documents in Firefox 55+, as no other browsers support it.
  • Support for the obscure MSThemeCompatible value of the <meta> element's http-equiv attribute has been removed from Gecko. No other modern browsers support it, and it was causing compatibility problems (Firefox bug 966240).



  • 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 (Firefox 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 (Firefox bug 1348409).
  • The HTMLFormElement.requestAutoComplete() method has been removed (see HTMLFormElement) (Firefox bug 1270740).
  • The non-standard, Mozilla-specific, WebRTC offer options mozDontOfferDataChannel and mozBundleOnly have been removed from the RTCOfferOptions dictionary and are no longer supported by RTCPeerConnection.createOffer() (Firefox bug 1196974).
  • Support for the proprietary Firefox OS Audio Channels API has been removed from HTMLMediaElement and AudioContext (Firefox bug 1358061).


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

Changes for add-on and Mozilla developers


Older versions