Review for Extra Folder Columns by Markus Gaugusch
Rated 4 out of 5 stars
Great thing! I'd like to report one bug, though:
I'm using the unified inbox feature, and when I install extra folder columns on thunderbird 3.1, all my Inboxes are named "Inbox", which is quite confusing ... without e.c.f. installed, they are named after the account they belong to.
To create your own collections, you must have a Mozilla Add-ons account.