Firefox 39 for developers

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Firefox 39 was released on July 2, 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


All devtools bugs fixed between Firefox 38 and Firefox 39.


  • Support of the <string> type on list-style-type, as well as its shorthand property list-style has been implemented (bug 1144607).
  • CSS Scroll Snapping has been implemented (bug 945584 and bug 1138658).
  • Cascading of CSS Animations and CSS Transitions has been rewritten to match the latest specification (bug 1125455).
  • Support for vertical scripts, with the writing-mode is now turned on by default on Nightly and Developer Edition, but not on Firefox Beta and Firefox Release (bug 1099032). Note that implementation is not yet complete and some widgets, like tables, will not obey to all values.
  • CSS filter acts like position: relative + overflow: hidden (bug 1125767).


  • A new ARIA role, switch, is now supported (bug 1136563).
  • Support for <link rel="preconnect"> allowing to anticipate a future connection without revealing any information has been implemented (bug 1135160).




No change.


No change.


No change.



  • Support for SSLv3 has been completely removed (bug 1106470).


No change.

Changes for add-on and Mozilla developers

No change.

See also

Older versions