Automated Attack Monkeys, Scalpel Mines, & Acid

My mum is having a weird firefox problem. It crashes when she tries to look at a shopping basket on any internet shopping site. It totally shuts itself down and you get the "firefox has encountered a problem" thing. Two virus checkers have found no threats, and my instinct is it's something she's done to the security settings. The problem is that mum is not the most computer literate person in the world, and trying to get her to describe the problem was difficult enough; trying to get her to tell me if she's clicked a security option will be impossible...

I have checked everything I can think of, and Mat can't think of anything. Have any of you lot got any ideas?

Mat has (kind of) mended my old desktop which he broke and I thus have internet access. Clunky, annoying, slow, crappy, internet access, but internet access none the less.

So now all I have to do is go through the 154 unread emails... Oh sod it, Question Time is on.