Press the little button in the titlebar that makes any window fill the screen.
Note that tabs will appear in the titlebar when maximized on Windows only, for Firefox 4, and only if you have the menu bar hidden. (It's hidden by default in Windows Vista and 7.) Tabs in the titlebar are planned for Mac and Linux too, but weren't ready for this release.
Re:Firefox 4 is here
by niels_olson on Tuesday March 22, 2011 @01:40PM
(#2355416)
For whatever reason, the voters seem to think I didn't try that before commenting. FF4 does not behave as described on my WinXP box at work.
Hm, don't know what to say. Perhaps disable the menu bar. That did it for me.
Here's what firefox 4 looks like running on windows XP: http://img855.imageshack.us/img855/460/20110322144851800x600...
edit: "this" appears to be as yet undefined. Unless ... can someone point me to whatever "maximize" is?
edit: FF4 does not behave as you describe on my WinXP box at work.