After updating to Firefox 3 many of my installed Add-Ons did not work. Some authors caught up and uploaded new versions ASAP but some seem to have lost interest in their work.
If you’re in real need of the plugin or are accustomed to it you may want to try this hack:

1. Download the plugin to your system (use an alternative Browser)
2. Replace the “xpi”-extension with “zip”
3. Extract the “install.rdf” file from the archive
4. Open the file with your favourite editor
5. Locate the <em:maxVersion> tag
6. Replace the version number with “3.0”
7. Save the file and store it in the archive (replacing the old one)
8. Rename the file’s extension back to “xpi” and install it in Firefox 3

Hopefully the plugin now will work with Firefox 3, i.e. Linkification and keyconfig do, whereas TabMixPlus did not work (but there is a Dev Build).