NEEDLESS TO SAY, THIS MADE ME VERY ANGRY.
To top it off, if you want to CHANGE THE SEARCH in FireFox, it's pretty obscure. I did, eventually find THIS which walks you through the process (ironically, it talks about going from Google to Yahoo instead of rescuing one's system for a Yahoo hijack!), which works like a charm.
My first though was "how can Yahoo think it's a good idea to hijack people's browser settings?" but then I remembered some of the MBAs we used to have to deal with back in my P.R. days, for whom "ethics" were in the same category as body fungus and halitosis. I'm sure there's some Brand Manager out there who thinks that it's brilliant that Yahoo Search is supplanting Google on all these computers. I hate to tell you how many times we had to risk an account to "go to the mat" with scumbags like that to convince them that the HATRED that their "tricky move" engendered wasn't worth whatever extra traffic it generated!
Anyway, thought I'd pass along that link so that you can fix FireFox after AVG/Yahoo screws yours up too.