Im switching to Camino for a bit, which is OS X native but built around the Gecko rendering engine. Its new release has a small feature that I love in FF even though its nothing major. When you add a pic to a post the pic appears as an image while youre composing the thread instead of being a URL.

It uses less CPU, doesnt support many extensions which doesnt matter to me and if anything seems a bit quicker.
FF3 hasnt done it for me at all with its leaky memory and maxing out CPU usage for the sake of looking a bit more Mac like. Tried it for a couple of releases but that CPU thing just bugs me.