Firefox layput issue, but fine in IE7

[ross mcivor]

I have a layout of sevral div's placed around the page I have a div that is ment to float behind my menu and content elements of the page but is seams if i put a height on (CoreClientContentOuter) in CSS document Core.Layout.Demensions if i put 100% its fine in IE7 but not firefox, and if I give a height then IE7 floats my footer infrount of my menu and content but doesnt its height into concidration but in Firefox its Fine, I'm very baffled.

Please help

magneticfields.hopto.org is the site in question.