How do I fix this frustrating mess? (Computer/Firefox help)

Computer nerds geniuses needed!

I was recommended this program by another user, so I assumed that it was legitimate and that it wouldn’t pull this kind of crap. However, it installed a toolbar and changed my default address bar and toolbar search settings to Yahoo without asking me for my permission, though I’m sure it was in the fine print. Even after I uninstalled the program (when it didn’t even work for what I needed), I can’t uninstall the toolbar and it keeps overriding me every time that I change my search settings. It will last for a little while, and then revert to what are now my default settings.

I am trying desperately to return to Google and fix this whole mess. Please help! I’m using Firefox. Thanks!