IE5Mac is a curse and shouldn't exist. You could consider letting them know that it doesn't work w/ IE Mac (that's what I'd do, 'cause I hate it). On many sites it's something less that 5% of the user clients anyway. I'd even consider doing it to IE 6 Win users just on principle
unfortunately, they're still around 80% of the user base on many sites.
Otherwise, have you tried setting a fixed width and height for this line, just to check if it's that?
<object type="application/x-shockwave-flash" data="forcasting_gates_receipts.swf" width="100%" height="83%" >