Idea regardingFirefox
Currently, the behavior of Firefox while in fullscreen means that as you click X on a tab to close it, it hides the navbar and fullscreens your current webpage, requiring you to move your mouse back up to the top of the screen to show the tabs once again and to close another tab.
To reproduce this, open a webpage (cannot be the about:newtab, e.g. google.com will suffice) and open many more new tabs, staying in your webpage (in my example, google.com) go fullscreen and click the Xs of the new tabs. Often you'll see your navbar disappear in an annoying manner.
A solution would be to make sure that the cursor is outside of the area occupied by the navbar before resuming standard fullscreen and hiding the navbar.
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