There are several possible issues to this...
1. Security in Mozilla. Is there anything specific to these pages that may be blocked out? File downloads, images, etc...
2. Find out from the person who wrote these pages if they've been tested in Mozilla. To my knowledge, which isn't very large where Mozilla is concerned (just learning it myself), there isn't specific html changes. However, when moving from one browser to another, they all have different nuances and if they haven't been tested, it is possible that a tag which works perfectly in IE will not function in Netscape, Mozilla, or other broswers.