Review for Extra Folder Columns by Eddie
Rated 4 out of 5 stars
Seems to do the job nicely.
One small installation quirk on Windows XP: after installation of the add-on, you are prompted to restart Thunderbird by clicking a restart button. This closes the Thunderbird window but does not kill the process, so Thunderbird does not appear to restart. After clicking the restart button, you need to kill the thunderbird.exe process using Windows Task Manager, then start Thunderbird manually. Don't know whether this is an issue with this add-on or with Thunderbird generally.
To create your own collections, you must have a Mozilla Add-ons account.