Rated 4 out of 5 stars

For those who can't get this to work on Thunderbird or Postbox, try this trick, which worked for me with version 0.18 running on Postbox 3.03:

1. Rename the .xpi file to .zip
2. Extract the zip file
3. Edit the install.rdf file, find your application (Postbox, etc.) and change the Max Version as needed (e.g., 3.*)
4. Re-zip the files into a .zip file
5. Rename the .zip file back to .xpi
6. Install into Thunderbird or Postbox