Gina at lifehacker has compiled and written a great little extension for gmail that you can get from her site.
To me so far the best part about this Firefox extension is the fact that all of those mailto: links on pages can now become gmail email links so that I can have work stuff for work and use my gmail to email much more efficiently. How does this work? Well if you have the extension installed you will see a little gmail envelope beside any links to that are email links on a page. Click on the gmail icon and it will being up a new tab with the compose email window for gmail and the email address already populated in the to: field