'Custom XUL keys' generates unclosable window on Linux

The window even persists after Firefox is closed!

keybinder 2.05
Firefox 53

Linux Min 18.1 x64 Cinnamon.

My title was mistyped and misleading. I've corrected it.

[Lord-Kamina]

I cannot say for sure but this is most likely caused (I think somebody reported a similar happening earlier) by something in the GUI/Window Server/however it's called that your version of Linux is using.

The Custom XUL window should have essentially the same parameters as the Mappings one, that is, a chrome window that behaves as a dialog.