No Horizontal Scrollbar

I've been using sheet for a while now on a windows XP machine with SP3, using the latest version of Firefox. No problems at all.

I recently bought a W7 laptop and have realized a problem. When using FF on my W7 laptop, I lose the horizontal scrollbar at the bottom of the sheet. (screenshot attached) When using IE8 on the same machine, the scrollbar is there.

Any suggestions? All of my sheets run into many columns and the scrollbar is very necessary for me.

Many thanks for any help that can be provided!