...start with the addons. I'm only assuming that the op wasn't running anything else. I've had a problem like this and it did turn out to be an addon. Other than that I've never had any problems with Firefox. It's still a great browser for me. You should also try to add more memory. Make sure your system is clean, no viruses, spyware, etc..
For anybody that's mentioned asking a question of Firefox support and never received a response, go there and look at how many questions come in a day. This has happened to me before. I asked a question which was never answered. It was frustrating. The way I understand it though, because FF is free and open source, the people that answer those questions are volunteers. There's only so many people to answer questions. If I'm wrong I'm sure someone will correct me. You'd probably get an answer faster on a forum. Also search, google search, forum search, firefox support search. Sometimes that's why questions don't get answered. They've already been asked.