Rated 4 out of 5 stars
It's easy to get this working in Firefox 4. XPI files are just zip files. Right click the white "Add to Firefox" button and "save as" the XPI file. After downloading rename the file extension to .zip. Then edit the install.rdf file inside, changing the version number from 3.7a to 4.2 or whatever. Save this in the .zip file, rename back to .XPI and then drag drop the .xpi into Firefox. Done.
So far the right click menu comes up with a transparent background, but everything else (well every function I use) seems to work. The menu is probably easy to fix, but I haven't tried.