TechniX

It is not highly recommended to manually modify/update Firefox  add-ons compatibility. However, if you really want to use your old favourite add-ons which are not upgraded to be compatible with the new version of Firefox, you still can do it on your own by this following way:

• Go to Firefox add-ons site where your favourite add-on(s) can be found (or simply go to Tools> Add-ons> Get Add-ons, and type your add-on’s name into the “Search All Add-ons” search field)
• Right click on “Install Now” and choose Save As… Your add-on should be saved as *.xpi file
• Now use Winrar (a popular shareware file archiver and data compression utility) to open the xpi file you’ve just downloaded. Extract only the file “install.rdf“, and then open it (remember this is just a XML file, so you can use any XML editor to edit it, even with Notepad).
• Look for this tag: “<!– FireFox –>“. Change the value of the tag “<em:maxVersion>” to your current Firefox version (e.g 3.0b4 for Firefox 3.0 Beta 4), then save it.
• Add this newly changed “install.rdf” file to the xpi file (by simply dragging it into Winrar windows). This will replace the old “install.rdf” file.
• Now use Firefox to open the xpi file. You’ll see there’s no error message saying that your favourite add-on is not compatible with the current Firefox version anymore.

Good luck and enjoy Firefox with tons of add-ons

Further reading: How to update add-on compatibility manually; Add-ons for Firefox 3.0 Beta 4