By default, the Firefox view item stays in the upper left corner. If I remove it and then try to resize the Firefox window, that space returns as empty and can’t be removed through customization. It goes back to normal when you close Firefox, but not when you resize it
I believe Firefox’s intended behavior is that the space shows up when the window is not maximized. I can’t entirely tell if that’s what’s happening in your video.
It’s the same thing when I maximize it back. It only disappears when I quit firefox, until I try to move the window
I’d try to replicate the issue with a new profile and Firefox Nightly, and if you are still seeing it I’d submit a report to Mozilla’s bugzilla.