I find that when the titlebar option is disabled in Customize options as shown above (comes enabled by default in LM), the top right corner buttons (min, max, close window) move slightly lower: a wider gap between them and the upper and right screen borders is created.
Here is the thing: in the rest of windows and apps when maximized (and in Firefox itself with the titlebar enabled too), it's easy to close them by just sending the mouse to the top right corner of the screen and clicking (the mouse gets to the screen limit and there still reaches the X button fine). So the issue here is that this lowering of the buttons forces you to actually "aim" the mouse at the close button, making that automatic action a bit more clunky and annoying.
Here I illustrate the issue with titlebar disabled:
This little bug has been present for years now on LM, but is still not fixed. Any idea where this issue comes from? I've gathered from other threads that the issue probably comes from Firefox's side, but being that this is a bug that affects a very common application (Firefox) in the main desktop environment of LM (Cinnamon), I think it would be worth it to be addressed by the Mint team. If I was a bit more savvy on this stuff I would try fixing it myself cause it's one of the very few quirks from Mint that still annoy me. I know pressing Ctrl+Q is suggested as an alternative, but the issue with this is that it closes *all* Firefox windows, not just the one you're in, so this is not an option if you have several Firefox windows opened (scattered in different workspaces for instance).