Why, oh why?

Why does my Web site look fine in Explorer and completely f'd up in Mozilla? I've tried to fix the glitch within FrontPage (yes, I know, I'm not exactly mind-blowingly tech savvy--get over it), but on certain pages all the formatting is screwed up. A big prize to the first person who helps me solve this puzzle. In the meantime, avoid viewing my site through Mozilla, superior though it may be. Sorry.