Your s/shot doesn't present anything that represents your issue. cntrl-n opens a new window. ctrl/t opens a tab which is added to the tab bar and then disappears with other tabs to present your full screen - does here anyway. So I'll check this as needing more info.

Sorry i meant new window

Hmm, that's interesting. With a maximized window, Ctrl+N gives you a new maximized window. But if the maximized window is in full screen, Ctrl+N gives you a new resizable window. Is this new? I don't use full screen enough to remember how it worked before.

I suspect an add-on could fix this, but I only have one that does the opposite (https://addons.mozilla.org/firefox/addon/change-maximized-to-resizable/).