This is hands-down the best add-on out there. If you have a laptop and your internet runs on a mobile broadband device like mine does, then your internet connection is probably pretty unstable.
My internet usually ends up showing me page errors, load errors, and connection errors about 50% of the time. However, with this add-on, It will simply continuously reload it until it works and goes through. Now I can step away from my laptop and not worry about having to come back to an error message in my browser!