Theres a few ways you can kill that. It probly got installed automatically from some site you went to... You SHOULD be able to go to your control panel on the start menu, then go to add/remove programs and then find it and uninstall it.

You REALLY need to scan for spyware and remove it for good. If you don't have anything try getting Adaware (http://www.lavasoftusa.com/software/adaware/). Scan with that and that should get it. It's also a good idea to have some kind of virus protection if you don't already for stuff like this. I would suggest AVG (http://www.grisoft.com/doc/Programs/lng/us/tpl/tpl01) its free and so is Adaware so...

Theres a few other things you can do to get rid of it but try these first. You'll probly want to close down your browsers while you scan. After closing all of them hit ctrl+alt+del, click the processes tab and search down the list for firefox.exe. If you see any of them still open, click them and hit end process to make sure theyre not hiding in the background...

If this doesn't solve you problem, let me know and we'll move on to the next step...

Good luck!