If I could install only 1 addon, it'd be this one Rated 5 out of 5 stars
There are many excellent mods for Firefox but this one goes a step beyond. It is not only convenient, it saves time, effort and worry - never again lose a long message you spent ages writing. Not only that, but I found it very useful when testing forms that had to be filled in repeatedly. Also - saves those usernames I can't be bothered to remember for supposed one off registrations on websites.