Firefox 29 for developers

Firefox 29 was released on April 29, 2014. 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

Major changes include:

  • Vastly improved web console - Arrays are shown inline without clicking to bring up in the right inspector, window objects show their url, etc.
  • Added the console API to Web Workers (bug 620935). Now you can log messages to the Web Console from Web Workers.
  • The Network Monitor tool now shows performance statistics using pie charts (bug 846599).
  • On the Inspector, preview tooltips of CSS transforms are now available (bug 726427).
  • DOM elements seen in the debugger and console can be removed or inspected directly, via the new buttons to the right of the variable listing.
  • A CSS source map is now supported by the Style Editor (bug 926014).
  • Autocompletion of CSS properties and values has been added to the Style Editor (bug 717369).

See the Mozilla Hacks blog post for details and other smaller changes.


  • CSS variables have been implemented (bug 773296). Mozilla Hacks article can be found here. They are enabled by default only for non-release builds (on release builds flip the pref layout.css.variables.enabled to true if you want to play with them).
  • Flexboxes now support visibility: collapse (bug 783470).
  • The box-sizing property has been unprefixed (bug 243412).
  • The will-change property, a hint to that something will animate has been added. The preference layout.css.will-change.enabled must be switched to true to enable it. (bug 940842)
  • Scientific exponential notation, like 3e1 or 10e+0, is now supported for <number> values and derivatives, like <percentage> and unit values, but not <integer>  (bug 964529).
  • Images of type <gradient> are now supported in border-image (bug 709587).
  • The touch-action property has been implemented. It is not activated by default; the layout.css.touch_action.enabled pref controls it. (bug 795567)
  • Remove redundant default style for <pre> element from quirk.css (bug 948914).
  • CSS Variables fallback incorrectly implemented (primary cycles)  (bug 950497).
  • @supports conditions with tokens after a declaration's priority should evaluate to false (bug 909170).


  • <input type=color> and <input type=number> are available by default.
  • Support for the non standard <pre cols> has been removed, as well as the layout effect of <pre wrap> . Both effects can, and should, be achieved using CSS. (bug 949879)




No change.


No change.


  • The CSP 1.1 experimental hash-source directive has been implemented. The preference security.csp.experimentalEnabled should be set to true to enable this functionality (bug 883975).

Changes for add-on and Mozilla developers

  • Australis and add-on compatibility - This is a major Firefox theme change that affects most extensions that involving the Firefox user interface.
  • nsISecurityCheckedComponent has been removed (bug 794943). Most consumers can simply remove nsISecurityCheckedComponent from their interface definition and they will continue to work.

Non-Australis changes TBD.

See also

Older versions

Document Tags and Contributors

 Last updated by: Sebastianz,