Strange Header Format

[Barkley Hound]

I hit some key combination by mistake and all of a sudden the menu bar vanished and the tabs appeared at the top of the page. There was a down toggle on the tab bar and when clicked the bar expanded and showed a preview of all the tabs. I ended up hitting the X to close Firefox and it turned out it was a new window and I returned to the original. What was I seeing?

[therube]

Can't say I know, but maybe check the shortcuts available in Tools | Web Development?
(I'm looking at SeaMonkey, FF might be slightly different location?)