When I open with a recent Firefox the search popup on the index page, I get a scrollbar because the height is few pixels to short:
This is not the case for the view of any topic:
This seems to be a bug! 
Kind regards!
Weird… I can’t recreate this on my sandbox, using latest Firefox (v43.0.4)
I can’t access Meta using Firefox at work, so I can’t fully test that, but it definitely isn’t a bug, more of a UX annoyance.
I can’t reproduce it here using Firefox 44 on Ubuntu Gnome 15.10.
I can - but, I never browse this way
There’s got to be something going on that makes the browser think there’s less room than there is.