Review for Extra Folder Columns by Henning Spruth
Rated 4 out of 5 stars
Very useful extension, I don't quite understand why this feature was removed to begin with.
I was able to make it work with the production version of 3.0 by downloading the .xpi file, unzipping it, editing the install.rdf file (change the maxVersion in line 14 to 3.0), creating a new XPI, and installing it. Do this at your own risk, of course.
To create your own collections, you must have a Mozilla Add-ons account.