I made a webpage for my dad using the navigation bar located here:
http://dynamicdrive.com/dynamicindex1/topnavbar.htm

And a style sheet.

I built it in Explorer, since that's the only browser I have. Someone told me that the menu bar and the top image (referenced from the style sheet) don't show up in Netscape or Firefox browsers.

Does anyone know how I can adjust either or both of the nav bar and the style sheet so that these show up in the other browsers?

yup. definitly not appearing in Firefox.

since it is distributed by DynamicDrive, using their forum area may hold the best result or use the email link to the creator from your linked page to ask him directly how to adjust his script to accomodate Firefox.