Check Multiple GMail Accounts

If you have more than one Gmail account and want to check them simultaneously, install Gmail Manager, a Firefox extension that allows you to manage multiple Gmail accounts.

Gmail Manager displays account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets. You'll also get a compose menu for your Gmail accounts.

Also read:
5 Fast Ways To Check Your Gmail
Gmail Skins: Change Gmail Interface
Gmail Horror Story: Gmail Account Deleted