Please see the the screens below for comparison:
Firefox and Safari -
Also, the 'Inquire' buttons on the below ad spaces flanking the main body do not work (should open contact page url in same window) in IE8 either! Any clue how I could fix both issues?
I edited your screenshots so they stopped breaking doctype.com.
In answer to your question you have float drop. Try adjusting widths and add borders so you can see which elements are making your ul columns too wide.
it might simply be a rounding issue.
Thanks. had someone more knwledgeable take a look. Made some css edits as you suggested.