I was a die-hard Safari user for a long time, only dabbling in Firefox, Omniweb and Camino.
Camino is neat but is missing too many features and I haven't seen that is extensible like FF.
In the past, I never cared for FireFox's unMac-like UI and because I use Safari Enhancer and Saft on Safari, missed that functionality on FireFox.
When FF 2.0 was launched I re-explored it, looking long and hard at available FF extensions.
I also found the Intel-optimized version called BonEcho with pre-fancified widgets (choose Aqua or Firefoxy).
Here's my current set: (some duplicate functionality of others and will eventually go)
1Passwd (indispensable keychain integration for multiple browsers) Worth checking out, IMO.
download them all
free ebook search
inline google definitions (replaces os x dictionary search)
search google images
tab mix plus
termblaster with Firefox
user agent switcher
GrApple (EOS) theme.
IMO, with new themes and extensions, FF is now a solid replacement for Safari in most, if not all areas.
Admittedly the UI is not up to Mac standards and there is no access to services (a big issue for many) but the functionality sets are about on par.
FF even has some neat tools not available on Safari. For example it has the Scrapbook extension and Zotero as well (both good for online research). I nearly forgot to mention that FF can scroll through the available tabs by placing the cursor on a tab and scrolling (even works on a portable with the track pad scroll feature) sweet!
I live in the export as pdf function of Safari (PhD student) and developed an effective workflow for saving relevant research articles using Safari.
With FF 2.0, I don't have the simple "export as pdf" option from Safari Extender, but I can use scrapbook or, given the proper type of website, Zotero. (I know FF can save as pdf via the print menu, it's not as convenient however).
I have found that FF works on sites that are problematic for Safari, including things as trivial as log out buttons, simple scripting etc. OTOH, I have never found a website that FF couldn't handle but Safari could.
I also use SpeedDownload and the plugin is a bit wonky in FF. There seem to be trade offs but the functionality gap seems to be closing between the two.
For me, the bottom line is that I could get by using either browser. I much prefer Safari's UI, services integration and generally prefer the available add on functionality (though it is commercial and FF extensions are mostly free). I am currently giving FF a go, searching out solutions when the need arises.
The only issue I've had with FF was when using the Google Sync extension, it stalled the browser big time. Since removing it, all has been swell.