Rated 4 out of 5 stars

This appears to work for newer versions of Thunderbird (I have 2.0.0.12) - if you can persuade it to install.

* rename the .xpi as .zip and unzip to a folder
* with your favourite text editor open install.rdf
* alter the em:maxVersion entry to a newer version (I entered 3.0+) and save
* re-zip
* rename the .zip as .xpi and give it a whirl in Thunderbird

Needless to say you should have a recent backup of your Thunderbird profile available for safeties sake.

The interface is a little basic; it could be polished up a little. But it is a simple tool that could be very powerful indeed.