When testing out new extensions one of the big pains is restarting FireFox after installing, uninstalling, or disabling an extension. RestartFireFox solves this issue nicely.
This may not be an issue for most people, but I have different profiles set up for FIreFox, and each time I start it asks me which one to use (I set it up this way). RestartFireFox bypasses this, and restarts the existing profile. Very nice.