Here's a website that can compare your page on any browser and version of said browser that you wish. You just ckeck all the ones you want to see how your page looks on them.
http://browsershots.org/
Looks like my web site does not load well for Firefox/Mac OS and Ubuntu, but looks fine on 100 something other browsers.
Gee, Linda, I use mac OS and Safari and/or Chrome for macs. Your website has always looked fine to me. So perhaps that browser compatibility website isn't very accurate for some browsers.
I think they meant using Firefox on a Mac PC.