If the dealbreaker is extensions, then I have to say that the only extensions I use in Firefox are Web Developer Toolbar and Firebug, which are excellent extensions. I only start Firefox to a) test sites on Firefox and b) use the aforementioned extensions.
Safari has a good built-in alternative to Firebug, which is available by going to Safari > Preferences > Advanced and checking Show Develop menu in menu bar. Then go to Develop > Web Inspector (or the other tools there.) Good for quick Javascript troubleshooting, as it also has a Javascript error/eval console.
In Firefox, I've found that other extensions usually make the browser more bloated and slower to use. They're nice, and sure, you could have ForecastFox tell you the weather, but it's not worth the bloat, in my opinion. And themes... :|
If you're like me, you'll miss a lot of extensions, but realize in the end that you don't really need most of them. del.icio.us was a favorite extension of mine. On Safari, I put the del.icio.us bookmarklet as the first bookmark on my menu bar. Then I hide the menu bar (apple+shift+b.) When I want to bookmark something to del.icio.us, I hit Apple+1, tag, save.
Firefox is a great browser, and Firefox 3 makes it even better, but Safari is a fast, no-frills, and great web browser. Although Safari has less 3rd party addons (and most of them superb, like Inquisitor) it's a web browser that does what you should expect it to do, and fast - browse the web.
It even has tab-completion, which makes it feel even more solid than what I've experienced with FF and various extensions.
I only open FF for FF-specific testing.
It's shareware, but it only nags when accessing bookmarks, not when saving them (which is all I ever used the FF extension for anyway...)
One feature that Firefox and Camino both have is the "Search for text when I start typing" which I use a lot for mouseless navigation.
I don't have any major gripes against Safari, I just prefer Firefox.
The GrApple Delicious theme for firefox is a must have for any Mac user.
This is really the main reason firefox is my default browser.
There's the "logout" link on this page, I just start typing l-o-g..., and once it gets highlighted, I can hit return and it'll click the link for me. Or if I need to go to the next word, I can still hit command+G. I don't have to invoke Find either, I can just start typing.
I guess the equivalent in Safari would be to cmd+f, type your word or phrase, press escape, then press enter. A couple extra steps but it does the trick.
However, it's got a lot of great plugins, so I use it for any questionable stuff and for proxied traffic, etc.
So far though, I am very unimpressed with the interface choices they made with Firefox.
One of the reasons I do this, in case you are not aware, is the IFRAME flaw, that allows a malicious web site to load, say, your bank's page into a hidden IFRAME, check to see if you are logged in and then - you guessed it.
(I just thought actually, could be the other way around, because vulnerabilities in Firefox are better known and more often exploited by not-so-trustworthy sites. So maybe it'd be smarter to use Safari for "the rest" and Firefox - for your banking, PayPal, eBay, webemail, etc.)
A lot of Greasemonkey plugins are pretty useless, but GoogleAutoPager is fantastic. It makes it so that Google searches are on an infinite page, as you scroll down the plugin gets the next page of results and slots them in so you never have to click next page
http://lifehacker.com/software/google/download-of-the-day-go...
Also, FF3 has Firebug. Game, set, and match.
I tend to hide the bookmarks bar because the way I use it, it just takes up unnecessary screen space, but the procedure above doesn't require it.
On the other hand, Firefox is definitely powerful and I love some of the addons, so I'll use it for development purposes - a lot of Firebug etc., have become moot with Safari's new and hot developer tools, but I still need to do some Firefox-specific things. But I don't want to spend so much time customizing how it looks/behaves.
Also, FF has extensions which although there are good alternatives on Safari, I sincerely doubt there are as many good quality ones for Safari as for FF.
It has very nice per-site preferences, really smart privacy options, plug-in management, etc.
One thing I hate about Mac in general is their hate for navigating solely with the keyboard. A lot of third-party apps do a good job letting you navigate using your kb only (such as cmd+1, cmd+2... to hit tabs in FF) but the last time I used Camino they were seriously lacking in that department. Clicking links with the mousewheel is also very important to me (to open in a new tab).
Who are these mysterious “they”?
In Camino, I use the keyboard to navigate, and my third mouse button opens links in new tabs, with no problems.