I prefer the Opera browser. It's UI is extremely customizable. Far superior to any browser on that front. The down side? There are a few sites that Opera doesn't work with. Most of them it probably would work, but they just refuse to allow any browser that isn't IE or Netscape/Mozilla/Firefox.
Opera's free version has an ad bar, but with broadband it doesn't affect browsing speed, and with a good resolution, it's not that obtrusive.
Sites that I know it doesn't work (or they just won't let you use it) are:
Gmail
Expedia (Microsoft owned - known for and sued for not allowing Opera to render properly)
There are more, but I don't remember them off the top of my head.
If a site renders in Opera, I use Opera. Otherwise Firefox is my tool of choice.
Opera's highlights are:
The best browser UI (User Interface) since it's the most customizable
Loads and renders faster than IE, Mozilla, and Firefox
Not only does it have tabbed browsing it has the best tabbed browser implementation
You can put a "Find in page" text box on the UI making the search of large pages extremely easy.
You can render any page any size by using a drop down box on the UI and it will not effect other tabs
You can tell Opera to tell websites that it is IE or even Mozilla to make them happy
F12 bring up a Quick Preferences menu for quick changes if a website is giving you problems
More popup blocking options than any other browser including opening only requested popups
Many more! Try Opera for yourself!