Gmail Manager creates a toolbar icon that can notify you of unread mail in multiple Gmail accounts and show you mail snippets. Last week's update to v0.5.7 created what Firefox labeled a "redirection limit for this URL exceeded" error when trying to access Gmail. Other Google Web programs like Calendar and Docs are unaffected.
Although frustrating, to correct the bug update the plug-in to the latest version, 0.5.7.1, which was published to fix the mistake. After you install it and reload Firefox, you should be able to access Gmail without going loopy.