Have you tried simpler Gecko engine based browsers? The fact that they use it on the Nokia Internet Tablets (considerably weaker than your laptop, I'd imagine) implies that you might have more luck trying something like Epiphany. Not sure if that particular project is terribly succesful, but it's a starting point. Opera, though not free, also warrants a try for the same reason. It tends to perform quite well on older and embedded platforms. That said, the first thing I'd try is installing FlashBlock and ad blocking plugins on your existing Firefox install. That generally does the trick for me, on these older machines. It's not the browser but the poorly designed/over-designed flash ads that tend to cause the vast majority of the problem.

Of course, there's always Lynx if the above doesn't work.

Sean Crago
Kathmandu, Nepal