1. Download the exention, rather than installing it.
2. Rename the .xpi file to .zip
3. Open and edit the Install.rdf file, change the MaxVersion to 3.*
4. Rename the .zip back to .xpi
5. Open about:config and create this preference if it doesn't already exist.
nb: It seams to update itself back to true whenever you restart firefox, but allows the installation to go ahead.
6. In firefox open the new .xpi file, and install away....