Awesome Firefox extension, install the plugin and browse the scripts. I installed a script that blocks all Yahoo ads. No more annoying flashing ads in my email account. There are tons of useful scripts. One of the script I installed compares the price of Amazon books with other online stores.
Also, install the scripts one at a time. Install a script and use your browser for sometime and see if everything is fine. This is important because I installed lot of scripts on my laptop and now one of the script is opening up all the links in a new window!
If you install it one at a time, you can easily find out which script is causing the problem.