Review for Gmail Manager by Michelle Butler
Rated 4 out of 5 stars
Works great, accept for not logging in.
I found a temporary workaround: do not set any accounts to automatically log in. Wait around 2-3 minutes after Firefox started, then choose "Login all accounts" from the right-click menu. Works perfectly most of the times (I sent this as a customisable feature request on the preferences page, until a permanent fix can be found).
To create your own collections, you must have a Mozilla Add-ons account.