Opera 8. Espically if you don't want to be downloading several extentions to get the features that Opera includes right out of the box:
Far superior tabbed browsing.
Paste and go.
Mouse Gestures.
Sessions.
Rewind.
A lot of Opera bashers cite the ad, but if you run a desktop resolution at or above 1152x864 (and arguably 1024x768) the ad height is smaller than your navagation buttons, and its over in the top right, where most people don't have buttons anyway.
I also find Opera's preferences to be easier to use and have more options. In Firefox you have to do about:config to get to many of the things Opera lets you play with.