The macosxhints Rating:
[Score: 8 out of 10]
One of the nice features of Firefox is its support for third-party themes. There's an extensive collection of themes available that let you customize the look of the browser. One minor issue I've found is that most of the third party themes seem to have scrollbar issues on OS X; the current exception to that rule (and my current theme of choice) is Abstract Mac. Abstract Mac has small icons (which I prefer), and since it's based on the OS X standard Firefox theme, the scrollbars work well. Themes can be downloaded and installed directly within the browser, though you'll have to re-launch it to activate a newly downloaded theme.
Just as Firefox supports third-party themes, Firefox also supports third-party extensions (153 of them as of today) that can greatly extend the feature set of the browser. As with themes, you install extensions directly within Firefox, and then manage them through the Extensions panel of the preferences dialog box. With 153 to pick from, you can probably find one to do whatever it is you think that Firefox should be doing :). Personally, I use Adblock (a powerful ad blocker), Flash Click to View (turns any Flash animation into a button that you click if you want to see it), and Download Statusbar (puts your downloads in a tabbed bar at the bottom of the browser window). Another powerful extension (though it can slow down the browser if you enable all its features) is Tabbrowser Extensions, which takes tabbed browsing to another level entirely -- for instance, you can rearrange your tabs by clicking and dragging.
NOTE: Some of the above URLs are directly pointing at the author's web pages; the easiest way to install any of these extensions is through the Extensions section of Firefox's preferences.
While there are still some inconsistent UI bits in Firefox, and the third-party themes need some help to work well with OS X, this is an amazingly powerful and fast browser. I can't say it's completely replaced Safari yet as my default, but it's only at version 0.8; by the time it hits 1.0, I suspect it may have assumed that role on my machines.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040224084528101