Review for Gmail Manager by AnotherHiggins
Rated 5 out of 5 stars
I've been using this extension for about 2 years. It is a must-have if you have more than one gmail account. very customizable.
Regarding the review entitled, "watch out":
On a new computer, I just installed it under FF 2.0.0.4 and it is working fine.
To create your own collections, you must have a Mozilla Add-ons account.