Anybody still having problems with their addons can try this:

1.) Download Nightly Tester Tools addon ( http://www.oxymoronical.com/web/firefox/nightly )
2.) Go to tools -> add-ons, right click on the add-on and select "Override Compatibility"
3.) Restart Firefox

I wish I could quote the website where I found it but I lost it, if anybody finds it feel free to post it.

Keep in mind of course that doing this might crash your Firefox if your add-ons are in fact incompatible with Firefox 3.