OK, I've tracked this one down. It's the tableborders class on the table surrounding the iframe that makes the whole thing completely black. If that is removed it looks great on Mozilla 1.1 (Linux).
Example:
<TABLE WIDTH=100% BORDER=0 CELLSPACING=0 CELLPADDING=1 CLASS="tableborders"><TR><TD ALIGN=LEFT VALIGN=TOP><IFRAME NAME=sbox ...
Change it to this:
<TABLE WIDTH=100% BORDER=0 CELLSPACING=0 CELLPADDING=1 CLASS=""><TR><TD ALIGN=LEFT VALIGN=TOP><IFRAME NAME=sbox ...