Comment 9 for bug 48654

I noticed this too. I think this is an X-mouse thing. I don't think windows
programs do this. Normally, in windows, scroll wheel events don't change the
focus (they're treated more like keyboard events most of the time).

There really should be some easy way of scrolling horizontally with the scroll
wheel, so either scroll over horizontal scrollbar or shift/alt+scroll (see
Issue 21925, Issue 17162, Issue 16736 and (more generally) Issue 18421). I
think implementing either one of these would probably stem the deluge of
feature requests for this kind of thing :P