Firefox 3 is released but most of the extensions are still not compatible with the new browser, here is a little hack to get your must have extensions work with the new browser, but before proceeding do keep in mind that this can be risky and may render your browser insecure, so proceed on your own risk, I tried it myself and everything works fine, All of my favorite addons like - Linkify, Rapidshare Link Finder, Remove Cookies etc. are working perfectly.
Follow the steps below, in-case something goes wrong a solution is also provided at the end of the post:
1. Type about:config in address bar and press Enter.
2. Now Right-click anywhere, Select New>Boolean, name the new config value extensions.checkCompatibility and set it to false.
3. Make another new boolean value as above with name extensions.checkUpdateSecurity and set the value to false.
4. Restart Firefox.
In very rare cases your browser might not start or crash due to incompatibility, simply start the browser in Safe Mode and revert back the setting or try enabling extensions one by one to see what works and what not.
Source : http://www.megaleecher.net/Firefox_3_Compatibility