I've been waffling back and forth between Safari and Firefox over the last few months. The flexibility of Firefox keeps drawing me close, like a moth to a flame, only to be burned because it doesn't "feel" right. Safari does. I've been slowly adding Themes and Extensions to make Firefox start to look/behave better, and it's close. Darned close.
The thing that might push it over the threshold for me was the addition of the All-in-One Mouse Gestures extension, which combines a bunch of stuff (including Mouse Gestures) into one nice package. In Safari (and every other Cocoa app on my system) I've been using CocoaGestures to add powerful mouse gestures to tasks like tab switching, closing tabs, etc... But on Firefox, I had to keep reverting to the keyboard (or moving the mouse to select a tab - much more efficient to just flick the mouse and have that motion translated...) Heck, I haven't even found a keyboard combo to switch tabs in Firefox yet...
The last two things that Safari rocks at are the integrated spel cheker, a handy key combo to put the text focus in the Google search field in the toolbar. I've tried the Firefox spell checker extension, with no luk.