Firefox 42 for developers

To test the latest developer features of Firefox, install Firefox Developer Edition Firefox 42 was released on November 3, 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


  • Vertical writing-mode is now supported with rtl scripts (bug 1131451).
  • The values of caption-side are now relative to the table, and changing actual meaning according to its writing-mode value bug 1202993.
  • Non-standard properties like -moz-margin-start are now aliases of their standard counterpart (margin-inline-start, …). Previously, it was the opposite. This affects the value returned by CSSStyleDeclaration.cssText and iteration of the properties in a style rule and iteration of the properties in a style rule, that both use the canonical form (bug 1118103).
  • The prefixed version of CSS gradients can be turned off by setting the layout.css.prefixes.gradients preference to false.
  • Several old bugs with float and margin collapsing behavior have been fixed (bug 478834, bug 538194, and bug 451791).





Service Workers

  • Request.context has been removed (bug 1188062).
  • The Push API has been activated by default on Firefox for Desktop (bug 1153499), but only on the Nightly/Dev Edition/Beta channels. It is being deactivated by default in the Release channel until consumer UX and debugging features have been properly implemented (bug 1207875.) You can still activate it by enabling the dom.push.enabled pref in about:config.
  • The PushManager.hasPermission() method has been deprecated in the spec, and replaced with the PushManager.permissionState() method. Firefox has updated its implementation to reflect this (bug 1183853.)
  • Service-worker-related additions to the Notifications API have been implemented (bug 1114554), but are disabled in this release.

Web Animations API

Our experimental implementation of Web Animations API have been extended to support:

Web Components

Our experimental implementation of the Shadow DOM has been modified:



  • The RTCPeerConnection.createOffer()'s options data type, RTCOfferOptions, has been updated to more closely match the specification. Among other changes, it is now based upon the RTCOfferAnswerOptions dictionary, which provides the voiceActivityDetection option.
  • The RTCAnswerOptions dictionary has been added. This is the type used for the options parameter to createAnswer().
  • The RTCICECandidatePairStats.mozPriority has been renamed to RTCICECandidatePairStats.priority (bug 1184426).

New APIs

  • The ImageBitmap interface and the createImageBitmap() method have been implemented. They are available on regular window scripts and in Web workers and allow efficient posting of images between window and worker contexts (bug 1044102).



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  • Firefox 41 and earlier versions were incorrectly accepting undefined or invalid pseudo-header fields in HTTP/2 responses. This is now fixed and the only pseudo-header field accepted from Firefox 42 is the :status as per the specification. Response headers containing arbitrary fields are considered malformed. (bug 1136727)



  • EV certificates with a validity greater than 39 months are now considered and handled as DV certificates (bug 1145679).

Changes for add-on and Mozilla developers



  • The TYPE_EMBED constant has been added to nsIContentPolicy to allow Gecko internals and add-on code to better differentiate different types of requests. Previously, TYPE_OBJECT was used for these cases (bug 1148030).
  • Similarly, the TYPE_SUBDOCUMENT constants has been split into TYPE_FRAME and TYPE_IFRAME (bug 1148044).


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JavaScript code modules

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Older versions