Our office is mixed Mac/PCs, and most of the Mac folks use Safari and the PC folks use Firefox. At home I've just started migrating from Safari to Firefox, and aside from a few form display issues I love Firefox. Right now my biggest complaint is that Firefox is still in public beta, so sometimes a site that doesn't work quite right in it will appear and function fine in Safari. However, Safari is also still pretty new, and M$ has bastardized Java and HTML so much that sometimes I still revert back to IE5 to make things work.
As a side note, Mozilla is working on a project called Sunbird, which comes in a stand-alone version and integrated version to Firefox (and a couple other of its browsers, I believe). What's cool about Sunbird (AKA Mozilla Calendar) is that when running with Firefox on PCs, it will allow PC users to access our iCal WebDAV server on our XServe and subscribe/publish calendars through it. So now we're all running ical or iCal (the open source or Apple's).