Gmail Manager Extension is an excellent application which allows you to check your e-mail directly from Firefox toolbar. You can manage multiple accounts and receive new notifications directly to your desktop instead of having to check Gmail all the time. The program displays account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets. You can specify at what intervals you want to check for Gmail and can customise exactly where you want to place the notifier either in your toolbar and statusbar. I already use a toolbar which can check my Gmail although I stopped using it a while back because it sometimes used to freeze-up Firefox periodically.