Firefox window reduced to title bar

While I'm using Firefox, sometimes (randomly; bad keypress because of fat fingers?) the window is reduced to the title bar.

After this, I'm utterly unable to return the window to its normal state.

I can't say if the culprit is Firefox (never happened with other apps) or the window manager (FVWM).

Distribution: Debian stable

Versions:

fvwm: 1:2.6.8-1+b1

firefox-esr: 78.6.1esr-1

More details required?

Many X11 window managers have a window shade feature which allows the window to be "rolled up" into just the titlebar. It is often bound to the mouse wheel, i.e. scrolling up or down while pointing at the window's title bar. It's also usually available in the window's right-click menu alongside the usual "Maximize" etc.