Rated 4 out of 5 stars

To get it to work with FF3.5:
Save xpi file to desktop (click ignore version check then right click and save link as). open xpi file in 7-Zip and edit install.rdf, change value to 3.5. Save and exit. Open Add-on window in FF and then drag and drop xpi file into add-on window to install.
Use at own risk! I've not experienced any problems but I only use double-click to copy tab (which works), the rest is untested.
Hope this helps.
(Only 4 stars due to FF3.5 hassle).