As with any list of Firefox tools, please do not install all of these at once unless you'd like to slow down your browser.
Better Gmail 2 - A Firefox extension that combines some of the most popular Greasmonkey scripts for Gmail into an easy to install extension. You can control which ones are operating by use of a simple checkbox interface.
Blank Canvas - Lets you create multiple HTML signatures that are inserted where you choose in your emails.
Email This! Bookmarklet Extension - Will send your chosen recipient the link, title, & highlighted text of the page you are currently viewing. Will work with multiple emails such as Yahoo and stand-alone clients, but is focused on Gmail (as if you couldn't tell from their logo).
Gmail Checker - Will allow you to check for new emails on traditional Gmail accounts and Google Apps accounts.
Gmail Manager - Beyond just giving you a visual and audio notification that you have new email, it will also allow you to log in to multiple Gmail accounts at once from the same browser. For more solutions to the multiple login problem, see Stan Schroeder's "5 Ways To Log Into Several Gmail Accounts at the Same Time".
Gmail Notifier - Allows you to keep an eye on multiple Gmail accounts as well as hosted domain accounts.
Gmail RTL - Adds the ability to write from right to left in Gmail, which is only offered in the Hebrew and Arabic versions of the service.
Gmail S/MIME - Allows you to send and receive S/MIME encrypted messages via your Gmail system.
Gspace - Have oodles of unused space in your disk quota on your Gmail account? Why not turn it into a virtual storage facility for files?
GTDInbox - GTDInbox turns your Gmail inbox into an organizational tool, allowing you easy integration between your mail and projects you are working on.
WebMail Notifier - While WebMail Notifier is not exclusively a Gmail extension, it is still an easy way to receive notifications of when you've received new emails.
WiseStamp - Allows you to create a much more dynamic signature for your Gmail accounts that will show off your social networking links, IM names and more. Can also set up business and personal signatures if you choose. See our previous review of WiseStamp here.
Xoopit - Xoopit will modify your Gmail search so that you can search your attachments, pictures and videos as well as the emails themselves. You can also add comments to pictures and pass them on to your blog or Facebook.