Basically xpi is the format that Firefox Extensions uses. Basically they are just zip files that are renamed.
So how do you get them to work?
1. Download the extension that does not work. (Right Click and save as, don't just click on the link, we don't want Firefox to touch it first)
2. Open up a Windows Explorer Window.
3. Go to Tools->Folder Options
4. Click the View Tab
5. DeClick the box that states Hide Extensions for known file types.
6. Right click your xpi file and delete the xpi extension and change it to zip.
7. Extract the file with your favorite ZIP client
8. Open the file named install.rdf in notepad
9. Find the line that states
10. Change the version to say 2.0 so it should read<em:maxVersion>2.0b1</em:maxVersion>
11. Rename the file extension to xpi<em:maxVersion>2.0</em:maxVersion>
12. Drag the file into an open Firefox window and install
Extensions that this works with
1. Fasterfox
2. FEBE
3. CLEO