With me Firefox (either 3.6 or latest) hasn't crashed for months, even though JavaScript is allowed everywhere. However, I use Privoxy for ad-blocking and I don't use Flash.
I use a simple but effective ad blocking scheme and allow flash only on two sites that I visit only occasionally. I disable iframes and Java. Perhaps that helps too. :)
I'm still using 3.6.x. I don't remember the last time Firefox crashed.
Darrell