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.
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