If I maximize Firfox its not possible to undo it with the same doublklick on the "header" (see picture)

I was able to fix it with the custom style thing but really badly so I hope someone can help me.

http://img6.imagebanana.com/img/rxlg8gkm/problem.png

Looks like you have gone into Kiosk mode rather than full screen.
Try pressing f11 to revert it to full screen.

Just for the heck of it, bring it up and try hitting the F11 key to see what happens. It probably has something to do with either the Firefox settings (if your using a beta version they can be buggy) or your desktops resolution is too small. When you first install an OS the desktop can be very small (640x480) and often you can't make it larger until you install the graphics drivers (safe mode loads as 640x480). Also browsers will often have system requirements that can include resolution. So if your maximum resolution is too small the window will go off screen. I have seen it before but I'm not familiar with the move function (you press some key or keys and get a circle with north south east west arrows inside) that can be used to right click and move the window down so you can manually resize it.

Try this
Firefox > Options > UNcheck "tabs On top".

This will move the tab bar BELOW the address bar and should allow for a double click.

Try this
Firefox > Options > UNcheck "tabs On top".

this will move the tab bar BELOW the address bar and should allow for a double click.,Cool, didn't know that. To add to this thread I noticed that some themes and personas conflict with the UI.

1690 x 1024 I don't think its to small. Also I am not using f11.

Again, if I double klick from small to big I can't undo it because the tabs are in the way -.-

But thanks for the help

Matthew, what was the point in giving me an unhelpful rating when I made a perfectly valid suggestion?
It doesn't really engender any willingness on our behalf to assist you further.