Safari is my every-day browser, it just gets better and better.
Firefox is the browser I use for two specific tasks:
I prefer to use Firefox for my daily Google Reader session, because it is so easy to turn off advertising. I would not need to do this if adverts in article I read were not so extremely obtrusive. I find it much harder to read when the text is surrounded by strobing adverts for products I have no interest in. So I just turn them off. Sorry for the publishers who need the income but the advertisers are doing you a disservice.
I use Firefox for debugging Ajax web applications, using the truely excellent FireBug. Firebug Lite runs in Safari, and it is an incredible acheivement, but it does not hold a torch to the real thing.
Firefox has lots of annoying niggles. The two that I find the worst are:
No obvious access to the system dictionary or spell checker.
Uses option instead of command as the meta-key to open a search or link in a new tab.
(As I said, I am used to Safari)
Anyway, it looks like some of my habits are about to change.
I just downloaded the latest nightly build of WebKit and it blew me away.
Webkit has something very similar to FireBug built into it now, I have not used it heavily yet, but what I have seen looks really promising!!
FireBug can give you some incredibly unuseful error messages sometimes, maybe WebKit will be better.
If you write and debug web applications for a living, I heartily recommend you try out a WebKit nightly, or if not, wait for Safari 4 which has recently been seeded to Apple developers.