Firefox 38 was released on May 12th, 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 37 and Firefox 38.



  • The <label> element no longer dispatches events to the label target element if interactive content is found between the event target and the label (bug 229925).
  • The <picture> element has been activated by default (bug 1017875).
  • The <meta name="referrer"> is supported for navigations from the context menu or via a middle-click (bug 1113431).




No change.


No change.


No change.


No change.


  • In Firefox, the autocomplete=false attribute is now ignored when dealing with a login form (bug 1025703). This is intended to encourage the use of more secure passwords by allowing password manager tools to work more reliably.
  • RC4 is now disabled when using TLS, except for a few specifically whitelisted Web sites. This whitelist is an interim measure until those sites are fixed (bug 1124039). This fallback is controlled by the security.tls.unrestricted_rc4_fallback preference, true by default for the moment (bug 1138882).
  • Web sites needing to fall back to an insecure version of TLS in order to work are now in a hardcoded whitelist which will shrink over time (bug 1114816). The whitelist can be disabled by setting security.tls.insecure_fallback_hosts.use_static_list  to false.

Changes for add-on and Mozilla developers

Add-on SDK


  • sdk/context-menu@2 was implemented (bug 1070952).
  • sdk/addon/bootstrap was implemented (bug 1075541).
  • sdk/windows/loader was removed (bug 970135).
  • Fix for default language detection on Linux (bug 1114712).
  • toolkit/loader now has opt-in module compatibility checking, which is enabled for all jpm add-ons (bug 1037235).

Electrolysis (E10s) updates



No change.

JavaScript code modules


  • DownloadTarget objects now have exists and size properties, allowing you to determine the existence of and the size of the download's target file on disk, as well as a new refresh() method, which asks that these values be updated.


See also

Older versions

Document Tags and Contributors

 Last updated by: teoli,