Firefox doesn't handle cookies for multiple, simultaneous logins to the same server.

Try one of these extensions for multiple cookie sessions.
Multifox:
http://br.mozdev.org/multifox/
Cookie Swap extension:
https://addons.mozilla.org/firefox/3255/
Cookie Pie extension:
http://www.nektra.com/oss/firefox/extensions/cookiepie/

Blame google for that one. They made the change back in Dec, but it didn't affect those of us who had cookies set. After the upgrade to FF 4, the cookies were outdated and required new ones, so we get the new behavior. So nice of them to ef it all up huh?

Google will no longer allow you to be logged into more than one account at a time (this includes your youtube account as well), unless you do it, painfully, through their kludge of a fix. Go into Google settings and turn on Multiple-sign on for each of your accounts (follow their directions). Then you can manually open any/all of those accounts at the same time in the same browser.

Note: 1. It's a crappy 'fix' to something they broke and apparently have no plans to fix.

2. It doesn't work with your youtube account. You will be automatically logged out of youtube if you switch your gmail log in to one not associated with that youtube account.

3. The official 'fix' for this is to install a different browser for each instance of a google id you want to run. FF, IE, Safari, Chrome would allow you to load 4 google ids. Brilliant no? No? No.

google sucks more and more every day.