If you need to, install 'IE Tab' plug in, that way you can call up the site using the IE. engine from within FireFox, you can even save the url in the add on so it automatically opens in the IE tab.
Erm, no, you don't.
Amazon works fine in an undoctored Firefox. He needs to fix whatever he messed up in his plugins/configs.
Firefox 15.0.1 works fine on Mac and PC for me.