Hehe...that is disappointing
but the hack is very easy and works like a charm...
- Download the .xpi file.
- Rename from .xpi to .zip/.rar
- Open the file in Winzip/WinRAR.
- Open file "install.rdf" in NotePad by double clicking
- Find maxVersion and change the maxVersion part from 2.0.0.* ...to... 3.0.1.*
- Accept Winzip/WinRAR asking to update the archive.
- Exit Winzip/WinRAR, and rename the .zip/.rar to .xpi
- Drag&Drop the file into FF3.
Bingo....you're good to go