Firefox Developer Edition
Release Notes

Release Notes tell you what’s new in Firefox. As always, we welcome your feedback. You can also file a bug in Bugzilla or see the system requirements of this release.

Firefox Developer Edition

May 15, 2015

Version 40.0a2, first offered to Firefox Developer Edition channel users on May 15, 2015

For more information about Developer Edition please see this MDN article and also this post.


  • Added protection against unwanted software downloads

  • New style for add-on manager based on the in-content preferences style

  • Improved scrolling, graphics, and video playback performance with off main thread compositing (GNU/Linux only)

  • Process separation enabled for opt-in testing

  • Graphic blocklist mechanism improved: Firefox version ranges can be specified, limiting the number of devices blocked


  • Kannada text does not display properly in built-in pdf viewer



  • Improved Performance tools in the developer tools: Waterfall view, Call Tree view and a Flame Chart view

  • New rules view tooltip in the Inspector to tweak CSS Filter values

  • New page ruler highlighting tool that displays lightweight horizontal and vertical rules on a page

  • Console API messages from SharedWorker and ServiceWorker are now displayed in web console

  • Inspector now searches across all content frames in a page

  • Removed support for binary XPCOM components in extensions, use addon SDK "system/child_process" pipe mechanism for native binaries instead

Web Platform


  • If Firefox is restarted from an add-on install notification, on-going private browsing downloads might be canceled without warning (1185294)

Get the most recent version

All Firefox Developer Edition downloads