Switching to Webkit isn't going to speed Firefox up. Gecko is very fast, especially now. As is SpiderMonkey. Firefox's biggest enemy when it comes to speed is add-ons -- which ironically is one of its best features.

Also, Chrome does definitely auto-update. It does it better than Firefox does (and I'm saying that as a Mozilla employee).

Give Firefox 8 (or 9 or 10 or 11, if you're into betas or nightlies) another chance, with a clean profile. I think you'll find it to be incredibly fast.