I am having similar problem... YAHOO snags the URL I enter then does a search rather than Firefox going to the URL... Google still shows as my search engine. How do I prevent this?

I found the problem. I did a search in the Forum using the word Yahoo (more specific terms relating to my specific problem did not work). I found a couple responses to the similar problem that talked about add-ons/extensions but (a) they were talking about the default home page when firefox is first started (b) I had already deleted the couple of extensions that I thought had anything to do with Yahoo. However after going through several pages I found a post that mentioned specifically (almost as an afterthought) that the Freeeze.com Net Asssistant extension started yahoo when the new page tab was hit. Well, I had not suspected that one but that turned out to be my problem. And I didn't even delete it (because it looks kind of important and I don't know yet what it does); I just disabled it and now I'm fine.