just a very quick read through your code and i think it may be because of the leftnav that is causing the problem. I think it might be because ie and ff do there margins and padding differenlty, and sometime ie can be like doulbe that of ff which in this case having 1em for the padding its now 2em which might be too big to fit in the space, so the content is having to be below it. Try making it less and see what happens.
This is only a quick guess so could be wrong. also i have no clue what size em is as i only use px as i find it so much easier.