Fix scroll keys in Firefox

When I fixed the broken <kbd>spacebar</kbd> key in Firefox (PR #3497), I apparently caused one serious usability issue as well.
*I'm amazed that neither I, nor anybody else, have noticed this until now. I only realized that this bug existed when trying to reproduce #3830.*

__Steps to reproduce:__
1. Load any PDF file, using either the *Firefox addon* or the *built-in version in Firefox Nighlty/Aurora*, for example: http://www.nada.kth.se/kurser/kth/2D1339/linser2005.pdf.
2. Click anywhere within the `viewerContainer`.
3. Attempt to scroll the document with either the: Up/Down arrow keys, Page Up/Down, Home/End or Spacebar.

__Expected result:__
The document should scroll.

__Actual result:__
Nothing happens.

__*EDIT:*__ Fixes #3832.