Firefox

Kiosk mode doesn't just force fullscreen, it disables right click, the tab and title bar, ...
Basically the browser is close to unusable until kiosk mode is ended, which I currently only know how to do via restarting Firefox.
And F11 is also disabled by kiosk mode. Interestingly, on the windows that were started before kiosk mode, it puts them into proper kiosk mode (after which F11 stops working of course).

Yes, this is more of a recovery operation. Whatever the fix may be, modifying the browser itself to open a window without decorations would be easier.

There are some usecases in which you really don't want to restart your browser.
The easiest way to update your kernel is to restart your pc, yet there is a market for live-patch kernels.
If someone accidentally infects their instance with kiosk, it may occasionally be preferable for them to follow a complex procedure to recover the instance, rather than doing the "simple" thing of restarting it.

Restarting may solve many problems, but there is a more difficult but less invasive solution almost every time.
Much like reinstalling may solve even more problems, but you can see that doing a reinstallation is not usually the right course of action.

SSB is still around, but also not what I was looking for. I just wanted a frameless window (and no other pwa functionality).
Fullscreen I disabled using my window manager. Under Linux you can commonly use alt+F3 to bring up the "right click on titlebar" menu, then disable fullscreen there. Generally ever window manager can disable fullscreen for windows, in a more or less accessible way (cough ms windows dll calls cough).

As mentioned below, This is recovery. I could ban kiosk mode to a separate profile, but unless you invent a time machine this won't undo having opened kiosk mode in an in-use profile.

I think the correct tool for my purpose would be something like Popup window, if you care about that part.