Review for Gmail Manager by gjmail
Rated 4 out of 5 stars
Great extension, that makes having several Google accounts easy.
Unfortunately, it still contains some minor bugs that haven't been fixed yet, as no release has been done for 6 months...
By the way, I don't think the problem we're having at the moment with Google Calendar is linked to Gmail Manager.
To create your own collections, you must have a Mozilla Add-ons account.