Rated 5 out of 5 stars

I could not get the steps to work after extracting the .zip, modifying the .rdf file, and re-zipping. When I went to install, I was greeted by a blank install panel after the .xpi began loading. I found a forum entry (modified slightly for this situation) that worked perfectly for me. Basically, the issue was with the re-zipping process - it doesn't need to be done.

1. Save the .xpi file to your desktop
2. Rename the .xpi extension to .zip
3. Extract the contents of the .zip file
4. Open the newly extracted folder
5. Grab the install.rdf file and put it on your desktop
6. Delete the extracted zip folder and its contents
7. Open the install.rdf file with a text editor (Notepad)
8. Change the Mozilla Firefox entry from 2.0.0.* to 4.0.0.*
9. Do a plain save so the file retains the .rdf extension
10. Go to your desktop (updated install.rdf and original .zip s/b there)
11. Drag and drop the install.rdf file over the .zip file
12. Click yes to overwrite the old install.rdf file
13. Rename the .zip extension back to .xpi
14. You should now be able to install the .xpi in Firefox

Modified from here (Ultimate Chicken entry):
http://forum.eeeuser.com/viewtopic.php?id=8926