Firefox 40 for developers

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To test the latest developer features of Firefox,
install Firefox Developer Edition
Firefox 40 was released on August 11, 2015. 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



Everything: all devtools bugs fixed between Firefox 39 and Firefox 40.



  • Prefixed rules (-moz-) for text-decoration-color, text-decoration-line, and text-decoration-style have been removed ({{bug(1097922)}}).
  • The property text-align now supports the match-parent value ({{bug(645642)}}).
  • In Quirks Mode, empty-cells now defaults to show, like in standard mode ({{bug(1020400)}}).
  • The -moz-orient non-standard property, used to style <meter> and <progress> element has been adapted for vertical writing-modes: the value auto has been dropped and the values inline and  block added, with inline being the new default value ({{bug(1028716)}}).
  • The property font-size-adjust has been fixed so that 0 is treated as a multiplier (leading to a 0 height for the font, hence hiding it) instead of the none value (leading to no adjustment, or a 1.0 value) ({{bug(1144885)}}).
  • Fix text-overflow doesn't work in vertical writing mode ({{bug(1117227)}}).


No change.



New APIs

  • The Push API has been experimentally implemented ({{bug(1038811)}}). Controlled by the services.push.enabled pref, it is disabled by default.

Web Animations API

Improvement in our experimental Web Animations implementation, mostly to match latest spec changes:


  • The CSS Font Loading API is now enabled by default in Nightly and Developer Edition releases ({{bug(1088437)}}). It is still deactivated by default in Beta and Release browsers.
  • The CSSCharsetRule interface has been removed and such objects are no longer available in CSSOM ({{bug(1148694)}}). This matches the spec (recently adapted) and Chrome behavior.


  • WebRTC: the negotiationneeded event is now also sent for initial negotiations, not only for re-negotiations ({{bug(1149838)}}).


Web Audio API

New extensions to the Web Audio API:

Web Workers


  • IDBTransaction are now non-durable by default ({{bug(1112702)}}). This favors performance over reliability and matches what other browsers are doing. For more information, read our durability definition.

Dev Tools


No change.


No change.


No change.


No change.


  • Using an asterisk (*) in a CSP does not include the schemes data:, blob: or :filesystem anymore when matching source expressions. So those schemes now need to be explicitly defined within the related header to match the CSP ({{bug(1086999)}}).

Changes for add-on and Mozilla developers


  • It is no longer possible to create transparent top-level windows {{bug(1162649)}}.

JavaScript code modules

  • Dict.jsm has been removed {{bug(1123309)}}. Use Map instead.


  • The nsIClassInfo.implementationLanguage attribute has been removed, along with the nsClassInfo::GetImplementationLanguage() function.
  • The following XPCOM interfaces have been removed; you should use the standard HTML interfaces instead:
    • nsIDOMHTMLBRElement
    • nsIDOMDivElement
    • nsIDOMHTMLHeadingElement
    • nsIDOMHTMLTableCaptionElement
    • nsIDOMHTMLTableElement
    • nsIDOMHTMLTitleElement


See also

Older versions