Review for Gmail Manager by PicAFlic
Rated 5 out of 5 stars
Works great now, version 0.5.7.1 is out:
http://www.longfocus.com/firefox/gmanager/releases/gmanager0571.xpi
To install, just drag the .xpl file in the Firefox window and it will install itself.
To create your own collections, you must have a Mozilla Add-ons account.