Tom,
Set the page compatibility to IE and Navigator. This will help.
FrontPage will try to do whatever you tell it to. There are also lots of
design considerations that will effect how things look. Things that FP will
have nothing to do with. Each browser behaves on its own a little
differently as I'm sure you already know. You need to know the little tricks
it takes to get all content to be the same for the various browsers. For
example, some browsers will collapse a table cell if it's empty (such as
Netscape and derivatives) so you'll need to put a transparent gif image in
there to shore it up. That's not a thing FP will do for you automatically.
So basically it's a two-part operation, adjust FP so it won't let you use
certain functionality that is definitely not cross-browser, and determine
what considerations you need to make for the other browsers and design
accordingly.
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - FrontPage