I'm guessing these videos are flash based? on my old machine, whenever firefox crashed it was flash related, have you tried firefox's safe mode?
firefox on vista hasn't crashed at all yet, had 2 minor problems but they are probably addon related
speaking of flash, everyone make sure yours is up to date, it's the best way to infect someone's computer if they have windows and browsers up to date, PSI secunia is a really good program to make sure you got everything patched and up to date