Created: 2012-03-23 04:39
Updated: 2016-12-26 13:10
License: other


Firediff is a Firebug extension that tracks changes to a pages DOM and CSS. This allows developers to examine the changes made in their debugging as well as those made by the page itself.


  • Monitor: Displays a log of all the changes made to the page. This can be filtered by Firebug and Application created changes.

  • Show Snapshot Displays a snapshot of the page's markup or CSS as a result of a given change.

  • Save Snapshot Saves a snapshot of the page's markup or CSS at a given point in time.

  • Unified Diff Generates a Unified Diff of the changes made to the page. The files generated by this represent the best guess match for the original source file and may require manual application.


Firediff 1.1 requires Firebug 1.8 and Firefox 6.

Method of operation:

To track changes to the DOM, Firediff uses the DOM MutationEvents ( to record all changes made to the page by both Firebug edits and application edits.

In order to track CSS changes, Firediff relies on the Firebug.CSSModule to record changes made to the page's CSS. As a consequence of this, Firediff can only track changes made within Firebug. Any CSS modifications performed by the application will not be recorded by Firediff.

Data Model:

Firediff maintains an internal model of the changes that have occurred within the page. This model is able to combine related changes into single change as appropriate. Each event implemented in the model supports apply and revert functionality on arbitrary trees.

Building From Source:

Firediff source is available at

From source Firediff can be run with or without Fireformat integrated. Without:

  • Checkout Firediff source
  • Register the project source directory path in the Firefox profile

When working in this mode no build scripts are necessary, but object to string formatting will not work unless Fireformat is installed as a separate extension.


  • Checkout Firediff source
  • Checkout Fireformat source from
  • In the Firediff directory create with an entry named fireformat_dir whose value points to the Fireformat directory
  • Run ant merge
  • Register the build/merged directory path in the Firefox profile

When working in this mode all functionality will be available but ant merge must be run after any changes are made to the source of either Firediff or Fireformat. This is the mode that Firediff currently ships.

Open Issues:

See TODO.txt or search the Firebug Google Code project for the firediff label. (

Cookies help us deliver our services. By using our services, you agree to our use of cookies Learn more