Or tab bar, whatever you call that one on top of Firefox window. I shouldn’t need this post, but every time when I want to take a screenshot of full window of Firefox, that is without any GUI of Firefox, but only the web page itself, I always went for guioptions, but the option of the visibility of tabline isn’t there, but in:

:set showtabline=never

The default value is always, there is also multitab for only showing when there are multiple tabs.

I am thinking that I could start training myself to get used to no tabline and no statusline at bottom, but I am not sure I am up to that, since there is no way to know how many tabs and what of them are opened at any given time.