I haven't got a definite solution for you but, as a FrontPage user, I do know that FP inserts some non-standard elements into the code it produces. <nobr> for example. These non-standard elements are dealt with differently by the various browsers, some more succesfully than others. The first thing to try is inserting a DocType as the first line of your code. You should do this anyway ... it's bad practice not to. Put <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> into your first line. It may help FF determine what to do with the non-standard stuff, it may not.
To really get it fixed you need to ensure all of your code is 'valid'. You can check this at W3C and here are your results.
Don't be phased by them but gradually deal with them one at a time and you page will benefit greatly.