Since the problem I'm having is likely a CSS problem, I thought I'd post this issue here despite being a "Firefox only" problem.
Anyways, I recently designed a new template for my site. Everything works just fine but, I just noticed tonight that the new template (which has a fixed width of around 830px) will not horizontally scroll if the browser window is made smaller.
This happens in Firefox only and only with this new template I designed. Other pages on my site, which use an older template, do not have this problem and will horizontally scroll just fine, if needed.