Firefox 28 for developers

Firefox 28 was released on March 18, 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

  • The console.exception property has been added (bug 922214).
  • The console.assert property has been added (bug 760193).
  • App Manager: a new Manifest Editor was added.
  • App Manager: the toolbox used for debugging apps is now embedded in the app manager UI.
  • Web Console: added a "split console" mode - press Escape to quickly open the console in any other tool.
  • Web Console: added a dark theme for the output.
  • Debugger: pretty-print minified JavaScript.
  • Debugger: simply hover over any variable or click on it to bring up a pop-up that displays the current value.
  • Inspector: added a color picker in rules view and various tooltips.
  • Browser Toolbox: allows add-on and platform developers to use almost all of the developer tools while targeting the browser itself.

More details in this post.



  • <input type=color> and <input type=number> have been implemented, disabled by default.




  • Support of the mathvariant attribute has been added (bug 114365).


No change.



  • Support of SPDY/2 has been removed.

Changes for addon and Mozilla developers

  • The interface of DeferredTask.jsm has been changed, and the isPending(), start(), flush(), and cancel() methods have been removed (bug 940408).


  • CSP was not enforced in sandboxed iframes. This has been fixed (bug 886164).
  • The CSP 1.1 experimental script-nonce directive has been implemented. The preference security.csp.experimentalEnabled should be set to true to enable this functionality (bug 855326).

See also

Older versions

Document Tags and Contributors

 Last updated by: Sebastianz,