It seems like Firefox 4, when it is finally released, will have its browser tabs above the URL bar. Google Chrome is probably the most known for this practice, though Apple did originally intend to have the same UI change in Safari 4. (Due to complaints during the beta period, Apple reverted the tabs to their previous position.)
Why is this so important? First of all, it’s different. So there will doubtlessly be complaints. Secondly, it fits with manila folder tab metaphor better. Alex Faaborg, a user experience designer at Mozilla, has a video explaining everything.
The release for Firefox 4 is tentatively scheduled for November 2010, though it’s certainly possible it could change.