why do my web browsers crash so much?

i have two macs, both of which i run up to date versions of safari and firefox on. both computers are essentially brand new as they both rec'd new hard drives recently so it's not like i've got anything weird in my system from years of web surfing. could it just be the websites? it usually happens on the busier sites like youtube, ebay, myspace, etc. but they can crash on ANY site. firefox can be brutal. i'll have a crash, reopen, and withing seconds experience yet another crash. everything else on my computers runs rock-solid. my connection was recently downgraded to dsl from high speed cable, but the cable was no better crash-wise. oh yeah, and i do frequently clear my history and empty the safari cache and reset it as well. not sure if that's supposed to help, but i figure it can't hurt either.