The script is a plug for Greasemonkey. It simply requires you to add your username and password to the file by installing then editing it. Do note that this should NEVER be used on publicly open computers or anywhere you think your account might be exposed. When I think about it, this is only for hard core nerds with top notch security on their private machines so unless you are one of us, do not use this!

Personally I use it on my private machine that I only got access to and that I never borrow or let others use, so it's pretty secure to use this if you also are that selfish with your own machine. ;)