Another way to force a vertical scroll-bar in Firefox is to use overflow-y: scroll; in the css rule for your
/* The rest of the body styles here */
However, while this looks and works fine in Firefox and IE8, if you look at the same page in IE7 or older IE browsers, you will see that you have a new problem, in that IE will insert a second scroll-bar - usually the same length as the run of the page content.
So to get round this you would have to duplicate your stylesheet - but without the 'overflow-y' style - and create a conditional style comment:
<!--[if lte IE 7]>
<style type="text/css" media="all">
which would need to be included in the head of each page.
You can see the effects of this method when the conditional css has not been aplied (if you have <= IE7) here but in Firefox or IE8 you will see that it has added the scroll bar track even where the page length doesn't require it.