One of the add-ons still not modified to work with Firefox3 is Zombiekeys. But I found that this inconvenience can easily be fixed. Download the xpi, and open it in your favorite archive manager. Extract the file install.rdf and find the line in the Firefox section that reads:
2.0.0.*
Change the 2 into a 3 and save the file. Now update the xpi with your archivemanager by putting the modified install.rdf back into it. In Firefox open the xpi-file (File->Open File, or CTRL-O). Restart Firefox to enable the add-on.