Firefox – How to Log in with Two Accounts in a Single Window

In Chrome you can use private browsing to achieve the same.

Chrome - Wrench(tools) menu/open new window without history

In Firefox you can use IE tab extension, and have another tab rendered with an IE engine to achieve the same effect.

Here are some other options: http://www.computer-realm.net/managing-multiple-gmail-accounts-with-firefox/

In fact, you don't even need multiple user accounts to accomplish this. You can execute several Firefox instances inside the same user account using the no-remote argument. The command line arguments are documented at http://kb.mozillazine.org/Command_line_arguments