Firefox 38 for developers

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



  • The <label> element no longer dispatches events to the label target element if interactive content is found between the event target and the label (Firefox bug 229925).
  • The <picture> element has been activated by default (Firefox bug 1017875).
  • The <meta name="referrer"> is supported for navigations from the context menu or via a middle-click (Firefox 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 (Firefox 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 allowed websites. This whitelist is an interim measure until those sites are fixed (Firefox bug 1124039). This fallback is controlled by the security.tls.unrestricted_rc4_fallback preference, true by default for the moment (Firefox bug 1138882).
  • Websites 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 (Firefox 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


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.


  • ";1" and ";1" now throw an exception if PR_APPEND is passed without PR_TRUNCATE (Firefox bug 1117580).
  • nsICompositionStringSynthesizer and nsIDOMWindowUtils.sendCompositionEvent() have been removed. Instead, use nsITextInputProcessor. (Firefox bug 917322)
  • nsIDOMWindowUtils.sendKeyEvent() is now deprecated. Instead, use nsITextInputProcessor (Firefox bug 1119609).

Older versions