On several occasions, full screen windows from a website could not display the relevant requested data because the height of Firefox's header plus the height of the website's window's header exceeded the physical display height of my HP laptop. If the headers trimmed off just a little of their excess space, then a portion of the data I wanted to see would be visible and so would their scroll elevators.
I've complained to one such website that I frequently use, but I never know in advance when I'll encounter the next window from the same or a different website. If Firefox trimmed off some of its own excess vertical space, then it would reduce the chance of encountering unusable windows on many other websites.
I took a screen shot of one such full screen window and used Windows Paint to circle the places that had excess space, including spots in Firefox's headers.