Tab Candy which is a feature of Firefox 4 will completely change the way we think of "browser tabs" in the future. This one innovative feature may push Firefox to the forefront over Chrome in user interface as a result.
Techrepublic does a good in depth review of Tab Candy showing screen shots on how it works and you can see this video demonstration on how Tab Candy changes the game for internet browsing:
Tab Candy allows you to drag out websites and group them in sections and categories on your browser space with little live images of the sites themselves, instead of having tabs where you have to click on the tab to see the site, you can group your sites into the same "tab folder" which sits on your browser page. It is almost like your browser becomes an OS desktop with each page becoming a folder or picture that you can open, close or organize together.
The ability to have groups of websites in the same "tab" are incredible and this will allow for increased productivity while browsing for sure.
One caveat is performance, Firefox already is losing in the performance battle against Chrome, Opera and Safari and having so many live sites in this format could further decrease performance if they don’t implement it right and improve the browser efficiency.
Still, I think tabs at the top of the browser bar are going to be a thing of the past very soon.