I've mentioned Nightly Tester Tools in passing as a way to force extensions to work in Firefox 2 even if they are "incompatible". Since the extension's site doesn't really explain how to use it, I decided to create an explicit step-by-step:
1. Install Nightly Tester Tools from here
, and restart Firefox.
2. Go to Tools->Add-ons.
3. Select the extensions that you want to make compatible and click Make all compatible.
4. Restart Firefox again, and you're done.