I can’t make out where the difference is; I have specified the exact height on content, sidebar, post and entry etc. But somehow on IE there is 1 (!) pixel height difference! Firefox shows the exact length, but on IE the content part has 1 pixel more height.
First, IE always displays things differently than real browsers, hence the need for IE hacks. (Try googling “IE hacks” to see what I mean.) This theme, for the most part, shows up beautifully for me in all browsers.
Second, please include a link to the page in question, as per the forum rules, so we can see what is happening.
You know your theme so well, maybe you can look once again?
From what I’ve read about CSS IE Hacks, it might have to be double or empty margins and paddings , please take a look only on content / content main / post and their clearfixes (lower right area), somewhere must be a double margin (with validation zero?) that can’t be handled equally by Firefox AND InternetExplorer …