Mozilla Firefox 4 has made it's way to the final release, and while most of the addons are compatible with the new update few still needs updation. There could be two reasons for an addon not to work with Firefox 4 - the first being it got functionally broken because of the new changes made in Firefox 4 (very rare), and secondly the developer has not yet updated/declared the addon to support the updated firefox version. Luckily, we have a fix for the second problem as shown below.

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.