The main differance we found was that tables need to be completely defined.
Tables embbeded in tables were being shifted in FireFox3
we now define the tables as:
<TABLE class=table1 cellSpacing=0 cellPadding=0 border=0 width="762" >
<TR vAlign="TOP" >
<TD width="144" >
<TABLE class=tableStandardb cellSpacing="0" cellPadding=0 border=0 width="134" >
<TR>....etc
This seems to have solved the problem :)
<TD