Increase biasing for Firefox's mousewheel events.

Based on testing (Mac, MBP, mouse wheel), Firefox consistently never
reports higher than -1 for 'single scroll events'.

Chart: https://gist.github.com/4692169

Other things of concern:

* The system of re-dispatching mousewheel events to a hidden div does
  not appear to be active on Safari/Chrome/Firefox - fails with a DOM
  Exception in all on my machine.