Review for Gmail Manager by ta_mosquito
Rated 4 out of 5 stars
I generally like it, but it often "freezes up" on me, not showing me new messages. I think there's nothing there but discover new emails when I go to gmail.
Also, with FF 2.0.0.11 on Vista (new computer) I've noticed it doesn't switch to the other account when there's a new message in the account not currently showing on the screen.
To create your own collections, you must have a Mozilla Add-ons account.