Rated 5 out of 5 stars
To fix this extension is very easy if you know what you're doing and not afraid to tinker a little.
Simply download a copy of the .xpi file. Open it up with any archive program such as winzip or winrar (it's basically just a zip file with the extension changed).
There will be a file in the root of the archive called install.rdf, open that up with a text editor. Line 8 of the file looks like this:
Next, change the line that says <em:maxVersion>3.7a1pre</em:maxVersion> to something else such as <em:maxVersion>4.5</em:maxVersion>. This allows the extension to work with Firefox versions up to 4.5.
Save this file back into the archive and close the archive.
Now load up Firefox 4.x, remove the existing extension (if there is one) and restart. Go to File->Open and browse to the location of our modified archive. Wait for the countdown and load it into Firefox...done.
Tested on Firefox 4.0RC1