There's really no reason to choose Firefox over Safari any more.
The only thing Firefox has on Safari is plugins, but the most useful of those, Firebug, is now essentially built into the new WebKit with it's excellent debugging tools.
The JavaScript performance is blazing too. Just check out the WebKit nightly: http://nightly.webkit.org/
+1 for Safari. Develop > Web Inspector is a very good Firebug replacement. I only ever open Firefox to a) test it in the browser or 2) to use more advanced Firebug features.
I tried to use WebKit debugging tools since I like Safari and Firebug is the only thing keeping me tied to Firefox.
Webkit Debug is ok for dom inspection, but it's horrendously bad for the on the fly CSS modifications that Firebug excels at. It's also missing the js eval console and several other crucial firebug features (+ yslow & firecookie). It's really a poor substitute at the moment.
For on the fly css editing, you can use cssedit2. It's alot better than firebug's css editing, although firebug still has a bunch of useful features (like showing the box model attributes of a DOM item). And CSSEdit2 use's safari as it's preview browser. I tend to use firebug way more than safari's debugging tools.
The main reason why i stick with firefox is dTa & AdBlock plus. Safari has some adblock extensions, but their awkward to install and just dont work as well as AdBlock plus. Linkification, tamper data (lets you edit your http requests), the bugmenot extension, PicLens and what not also rock too.
> looks like a ">" with 3 horizontal lines to the right
.. and to think that earlier in the Mac's history the mantra for GUI design was "a word is worth a thousand pictures" (see: http://www.asktog.com/columns/038MacUITrends.html). If Apple still followed that mantra railsjedi would have actually noticed that Web Inspector had all these awesome features.
Anyway, besides that, Firefox 3 doesn't seem to leak memory like a sieve like Safari.. the main reason I switched back to Firefox again.
If I use Safari for a few days, I can guarantee it'll be using 700MB->1GB of system memory and I'll need to quit and relaunch. Firefox 3 tends to hover around 200MB no matter how long I've been running it. That's a big deal, even with 4GB of memory.
If you can stand typing command-F you'll find that Safari has an excellent feature that very visually highlights the found text. It dims the irrelevant parts of the page and highlights the found text with a very hard to miss box. This is my favorite Safari feature.
I had used Firefox on my Macs for quite a while, mainly because Firefox had tabs while Safari did not. Even once Safari got tabs, I continued using Firefox, just out of habit.
Then one day, GMail refused to load in Firefox. I tried multiple times, and, while I never found out why this happened (a Firefox update? a GMail update? maybe someone here knows), I quickly ditched Firefox for Safari.
The only thing Firefox has on Safari is plugins, but the most useful of those, Firebug, is now essentially built into the new WebKit with it's excellent debugging tools.
The JavaScript performance is blazing too. Just check out the WebKit nightly: http://nightly.webkit.org/