I use the Developer Tools in Firefox a lot, and somehow they got stuck in full-screen mode, I cannot remember how I pulled that off. Nothing helps: double-clicking the title bar, Windows+ArrowDown, and the helper icons in the top-right corner have disappeared: 
All I can do is enter or quit using F12, or move the window to a different screen/workspace using Ubuntu keys. Restarting Firefox doesn't fix it, when I type F12 the window is maximized once again.
How can I deactivate full-screen?
Possibly useful info: Firefox Snap for Ubuntu, 131.0.3 (64-bit), canonical-002 - 1.0