Without looking at the page, you need to subtract padding and margins and make that the width so that IE doesn't cause a 'div drop'. You can either use that width or leave as is and use an IE hack or a conditional comment declaring the correct width for IE.
BIt late here and too tired to look through my book marks, but you could do a google for IE hacks or IE float drop (?) and I'm sure you'll find some pages relating to this....
I've learnt that I should be developing & testing with IE rather than with FF.
I think you've learnt wrong in that case. All the wisdom on Sitepoint says develop on FF because it renders the code correctly. Any adjustments to IE are because of the quirks (not quirks mode) of IE. You'll end up with more problems if you don't develop in FF and correct in IE.