Run old extensions in Firefox


Firefox 3.5 new iconMozilla Firefox 3.5 was released today. While this is great news for users and web developers, not all addon authors have got round to updating the addons for the new version.

In many cases, the extensions and themes do not break, but simply need to be marked compatible with the specific version of Firefox. In such cases, you can use the simple about:config hack below to disable the extension compatibility check and use older addons without any problems.

Note that this presents the risk of crashes and problems, so make sure you either know that your favorite extensions and themes are compatible with Firefox, or you have proper backups of your data/settings/bookmarks/addons.

Hack to use older addons in Firefox

Here are the steps to disable extension compatibility check to use incompatible Firefox themes and addons:


Enter about:config in the address bar to bring up the about:config page.

Right-click anywhere, point to New and then to Boolean.

Disable Firefox addons compatibilty check

Enter the key extensions.checkCompatibility and hit OK.

Run older Firefox addons hack

Choose the value as false.

How to use incompatible extensions in Firefox 3.5

If you type the first few letters of extensions.checkCompatibility in the filter field, you can find the value set as false.

Run older addons in Firefox 3.5

You have to restart Firefox for the hack to be in effect.

I found that the above hack worked in letting Firefox use the GrApple theme – which is still ages ahead of the default Firefox theme.


  1. Nightly Tools is not really easier, unless you’re a total beginner. This method requires one edit, and is easily understood by everyone with clear instructions.

  2. U simply rock, works flawlessly on firefox 3.5, using an old theme right now due to your config hack.


  3. tried this for unplug 2.003 add on and no dice. Didn’t seem to do anything do reset

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