For questions and problems with the CMS core. This board is NOT for any 3rd party modules, addons, PHP scripts or anything NOT distributed with the CMS made simple package itself.
i've something similar not with cms but with phpmyadmin + Firefox. I had a big database table (~500 columns and ~500 rows). With IE all the list was loaded normaly, but with firefox i was getting the donload dialog, offering me to download the listing php file.
I asume in your case, maybe it's because you're rewriting to *.php with htaccess.
I never had such a problem useing cmsms though, i'm using *.html for rewrites.
Yes, modrewrite only changes the name of file when showing in browser. But i'm saying that it depends on how Firefox handles php files (no matter it's only modrewrite or normal). In my opinion it's firefox.
Btw i visited your site with ff 1.5.0.1 and everything sees to work fine for me.