Call me a switcher. I've been getting more and more angry at Safari's lacks, and after Apple refreshed it slightly in the latest OS X upgrade, I'm still not impressed. Safari is very close to great, but they're not putting enough time and effort into the little things that just add up to irritation.
I cannot get Safari to run for more than a little while without getting interludes of spinning rainbows. I know all the tricks, including disabling favicon downloads, reducing page load delay settings, and so forth. Nothing helps. Firefox has a few things that bug me, including a lack of keystroke commands--I'm sure I just need to dig deeper to find them or set them to my needs--but the rendering is fast and no spinning rainbows. It's multi-threading (whether it's real or just a programmatic illusion) means that it can render in one window while I switch to another.
I've now switched to Firefox on both my home and work machines. Once I moved into full-time Firefox, as opposed to testing it, I like it even more. Everything is snappier, and simpler. You can't import bookmarks from Safari, which is silly and pointless.
Ironically, to make Firefox your default browser under Mac OS X, you have to run Safari, select Preferences, and set Default Web Browser to Firefox.