Rated 5 out of 5 stars
It will work with Thunderbird 3.1.1:
1) Download the XPI file
2) Change the extension of the XPI file to ZIP
3) Un-zip the ZIP file
4) Open the file INSTALL.RDF in a text editor
5) Find the tag: 3.0.*
6) Change "3.0.*" to "3.1.*"
7) Re-Zip all the files
8) Rename the ZIP extension back to XPI
9) Install in Thunderbird. Restart it.
OBS 1: If installed already, uninstall it from Thunderbird first
OBS 2: You can usually edit the RDF file inside the ZIP file without having to un-ZIP/re-ZIP. Most ZIP programs should allow that.
And yes: why isn't this feature/option part of Thunderbird by default????