Daring Fireball wrote a nice comparison between FF3 and S3, finding Safari to be the better contender. He makes a few good points (I like FF3 better than Safari, but it might just be for ephemeral, weenie reasons like not wanting to use something new) but this sticks out:
Tabs — Firefox 3 does let you drag to reorder tabs within a window, and drag tabs between windows, but it doesn’t let you drag a tab out of a window to create a new window with just that tab. Safari 3 does. Picky-picky, I know, but I use this feature in Safari every day to group related tabs together in their own window.
The rest of his comments are compelling as well: better key shortcuts, built-in dictionary, inline PDF viewing (all for Safari) and a cool robot icon for FF. What would you pick?