Firefox 63 for developers

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This article provides information about the changes in Firefox 63 that will affect developers. Firefox 63 is the current Nightly version of Firefox, and will ship on October 23, 2018.

Changes for web developers

Developer tools

  • The Fonts tab in the Page Inspector now includes an editor that makes it easy to view and edit the settings of the fonts on your page. See Edit fonts for details.
  • When you hover over an object in the Accessibility Inspector, the item is highlighted and its role and name will be shown in an information bar on the page. (bug 1473030)






  • Removed offset-block-start, offset-block-end, offset-inline-start and offset-inline-end; these have been renamed as described above (bug 1464782).


No changes.




  • Experimental WebAssembly Module IndexedDB serialization support has been removed (bug 1469395).


New APIs


DOM events

  • Handling of the Alt key on the right side of the keyboard has been improved on Windows. If the user's current keyboard layout maps the Alt key to the AltGr modifier key, the value of KeyboardEvent.key is now reported as "AltGraph". This behavior matches that recently introduced into Chrome (bug 900760).

Service workers

Media, Web Audio, and WebRTC

  • Microphone access now works simultaneously in multiple tabs, even within the same content process. This improvement applies to the regular version of Firefox as well, though web developers were probably most likely to run into this (bug 1404977).
  • RTCDataChannel has been updated to support the sctp-sdp-21 data format for the data, in addition to the older sctp-sdp-05 format previously supported.
  • Since it was deprecated in the specification anyway, the limited support for Doppler effects on PannerNode has been removed from the Web Audio API. The AudioListener properties dopplerFactor and speedOfSound have been removed, along with the PannerNode method setVelocity() (bug 1148354).
  • The ConstantSourceNode node type for Web Audio API now has a default channel count of 2 rather than 1, in order to match the specification (bug 1413283).
  • The Web Audio API interface AudioScheduledSourceNode (and by extension, all the other node types based on it) now throw the correct exception when a negative value is specified for the node start time. That error is RangeError (bug 1413284).
  • The minimum and maximum permitted values for an AudioParam object's value have been changed to the minimum negative single-precision floating-point value (-340,282,346,638,528,859,811,704,183,484,516,925,440) and the maximum positive single-precision floating-point value (+340,282,346,638,528,859,811,704,183,484,516,925,440), respectively (bug 1476695).
  • The {domxref("SourceBuffer.changeType")}} method, which allows changing codecs during an active stream, has been enabled by default. This is part of the Media Source Extensions API (bug 1481166).
  • The AudioParam.setValueCurveAtTime() method has been updated to correctly accept an array of floating-point values to indicate the parameter's values to change to over time. Previously, it was requiring a Float32Array (bug 1421091).
  • AudioParam.setValueCurveAtTime() has also been updated to correctly return a proper TypeError when a non-finite value is found in the values array (bug 1472095).
  • In addition, setValueCurveAtTime() has been updated to ensure that when the parameter finishes following the specified value curve after the duration elapses, the value of the parameter is set to the last value in the list of values to curve through (bug 1308436).
  • The RTCRTPStreamStats dictionary has been renamed to RTCRtpStreamStats for consistency with the other dictionaries and the specification (bug 1480498).
  • Support for the RTCRtpStreamStats dictionary's kind property has been added (bug 1481851).
  • The RTCRtpStreamStats dictionary's isRemote property is deprecated and will be removed in Firefox 65. A warning is now output to console when this property is accessed. See this blog post on the Advancing WebRTC blog for details (bug 1393306).

Canvas and WebGL

  • A new powerPreference context attribute has been added to HTMLCanvasElement.getContext(). On macOS this allows WebGL non-performance-critical applications and applets to request the low-power GPU instead of the high-power GPU in multi-GPU systems (bug 1349799).



No changes.




  • Site favicons are now subject to Content Security Policy, if one is configured for the site (bug 1297156).
  • Firefox now recognizes the CSP script-src directive's 'report-sample' expression when generating violation reports. This directive indicates that a short sample of where the violation occurred should be included in the report. Previously, Firefox always included this sample (bug 1473218).



No changes.



  • Corrected the behavior of PerformanceObserver.observe() to simply do nothing if no valid entry types are found in the specified array of entry types to observe, or if the array is empty or missing. Previously, Firefox was incorrectly throwing a TypeError (bug 1403027).
  • In OpenSearch, Firefox now accepts application/json as a search URL type, as an alias of application/x-suggestions+json.


Changes for add-on developers

API changes

See also

  • Site compatibility for Firefox 63

Older versions

Document Tags and Contributors

Last updated by: rachelandrew,