It allows you to add empty space to the Firefox UI if you'd like

Flexible space automatically adjust its width (hence flexible in its name). It can be used to center items on a toolbar. You can see flexible space in action around the location/address bar and search bar container if you make the Firefox window smaller and wider. Firefox supports fixed space as well, but doesn't provide it via customization mode.

Thank you Cor-el. You are wise and VERY helpful. That makes sense.