Peak Firefox UI was when the "close current tab" button was always in the same place and I could do it with muscle memory instead of hunting for wherever my current tab is on the list.
You probably know this, but Ctrl+W can be an alternative to close the current tab since those buttons won't move. Or using mouse gestures (blast from the past I suppose, but e.g. I use the 'up' and 'down' gestures to scroll to the top or bottom of a page very often).
