Firefox Gmail Manager (Add-On)

Gmail Manager is a Firefox extension that allows you to manage multiple Gmail accounts and receive new mail notifications. Displays account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.

Features
Manage multiple Gmail accounts
Receive new mail notifications
Support for Gmail hosted accounts
Ability to import/export account preferences
Tooltip with account details and new mail snippets
One-click access for loading your Gmail account
Mailto: links detection
Quick mail compose for Gmail accounts
Customizable icon placement in the toolbar/statusbar

More info and download.