It sounds as if the page you developed in FP is using one of the newer features available to web pages, but not "fully supported" in earlier versions of Netscape.
You should especially look for DHTML elements like animated menus, roll-overs and so on. You might also look into stylesheets which might be in use.
This might not necessarily be a "Netscape" problem. We who write web pages by hand (text editors), or those who develop truly useful development environments (like Dreamweaver, etc.) know how difficult it can be to develo what we call "cross-browser" technologies. This is especially true of DHTML and CSS-2. Frontpage, showing its heritage all too well, typically generates pages with MS Internet Explorer in mind. The DOM used inIE is not necessarily thatg used in other browsers, and therefore the code generated may not necessarily work in other browsers.
"How to write cross-browser functionality?"
It ain't easy! Basically, we test the client's browser, and try to apply the appropriate code. See "webreference.com" for lots more info.