In a sense, IE has the layout correct, and FireFox has it wrong.
The outer table is specified as 760px wide, the 100% table with the
navigation in IE is about 760px wide - in FireFox it is closer to 840px.
The main table has been dimensioned incorrectly - no account has been
taken for borders, cellspacing which are in addition to the width of the
cells. So the main table is wider than it should be.
Example - the outermost table has 5px borders, 5px cellspacing, and a
cell 760px wide. This is a total width of
760+5+5+5+5 = 780px
The table nested inside this has 4 cells, each 200px wide, plus 3px
borders, plus 3px cellspacing - this adds up to
836px - all inside a cell 760px wide.
It would probably be better to remove the dimensions altogether, and
allow the tables to form their own layouts. This will be consistent
across browsers, and if the outermost table is centred - it will be
centred across all browsers.
Also note that bordercolor is IE only, and the text has no colour set -
the text appears orange in my copy of FireFox.