In my search to get Google Tool bar working with Firefox 3 I stumbled across this utility called Nightly Tester Tools. It allows you to run out of date extensions/add ons with current versions of Firefox even if its not officially supported. To get this working just do the following.
- Goto here and click Install (v1.3b3)
- Make sure firefox has permission to install and do so.
- Restart Firefox
- Go to Tools > Add ons
- Click on the add on you want to get working
- Click " Make all compatible ".