Why? Firefox is just too slow. Remember the good old days when Firefox was the leaner, faster, less-bloated alternative to Netscape Navigator? Those days are in the past. Firefox now takes over ten seconds to show a window after I launch it, whereas Safari is up immediately. Safari is also faster at rendering pages, and at just about everything else. It takes up a fraction of the memory of Firefox, and has not yet started using 100% CPU for no reason at all, as Firefox tends to do.
The thing that Firefox has that other browsers can't match is the huge set of add-ons (extensions, themes, etc.). While many of these are really cool, there are really only two that I consider must-haves:
The lack of Adblock Plus is noticeable right away. Web sites that I visit frequently are suddenly awash in ads. It's very distracting. There are a couple of Safari extensions that supposedly work like Adblock Plus on Firefox, but they don't work as well. Most Safari users seem to be happy with simply blocking Flash ads with ClickToFlash, but I'm not satisfied with that. I've just started a trial of GlimmerBlocker, which seems pretty good so far.
PasswordMaker is a nifty little Firefox extension that automatically fills in password fields with passwords that are unique to each site, generated by hashing a master password with the site's name. This is great, because I haven't had to remember any passwords with Firefox, but it's bad in that I now have to teach Safari all those passwords, and I have to do a lot of manual copying and pasting.
With my new foray into web technologies, maybe I'll devote some time to trying to get these essential Firefox extensions ported over to Safari.
Aside from the missing add-ons, I also miss Firefox's keyword search functionality. There is a Safari extension, Keywurl, that supposedly provides this functionality, but it is not yet compatible with Snow Leopard. (If you search hard enough, however, you can find some unofficial 64-bit builds.)
Unfortunately, Safari does not actually have a supported plug-in/extension mechanism, so most of these extensions are hacks that can crash Safari or which suddenly stop working whenever there is a system update. It's a shame that so many products today are not designed to be extensible or user-programmable.
Aside from those little missing features, I'm happy with Safari. (For now, anyway.)