[Disclaimer: This article is a copy-paste from here: http://www.techzilo.com/run-older-firefox-addons-hack/]
Hack to use older addons in Firefox 3.5
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.
Enter the key extensions.checkCompatibility and hit OK.
Choose the value as false.
If you type the first few letters of extensions.checkCompatibility in the filter field, you can find the value set as false.
You have to restart Firefox for the hack to be in effect.