I haven't had a crash yet in FF5 or 4 (flash crashed several times but did not bring down the browser). Also, I notice about 0 rendering delay.
I don't know if crashing is the right word but it does become completely unresponsive often enough that it is noticeable and annoying. (Unresponsive as in it has to be killed to regain functionality.)
Likewise, IE9 ain't bad.
This explains that ;-)
I'd like to see such folk browse today's web with the original version of FF... :) The average webpage today is way more complicated than 10 years ago.