As a webpage author you already know this, too. But I feel like pointing out the obvious.
All web browsers try to emulate the standard, by which I mean IE. But it's difficult to exactly duplicate another browser's behaviour at the limit, where one begins to push the basic HTML elements into complex service.
In the early days, IE put a lot of effort into emulating Netscape layout. More recently, Mozilla has put a lot of effort into emulating IE layout. A more recent build of Mozilla may do a better job on your site than the aging Netscape 7. Netscape 4 is another beast, completely. If Mozilla is a modern human, Netscape 7 is a cro-magnon, IE 6 is a neanderthal, and Netscape 4 is an orangutan.
In the end, web standards aren't. In a W3 standards compliant browser like Mozilla, for tricky layout it's usually better to use style sheets, rather than tables. If you're really motivated, you may need to deliver different content, depending on the browser being used. It's a very common trick; the web is full of such sites.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7a) Gecko/20040220