Very rarely a web page will not render as it was intended when viewed in Firefox. This happens because most pages on the internet are created and coded with Internet Explorer in mind. Internet Explorer interprets code in a proprietary manner and in many cases to force a page to render correctly for Internet Explorer the page must be written in a manner that is not compliant with HTML and W3C standards. This means that a browser other than Internet Explorer that is extremely compliant with standards in its rendering of pages may occasionally have a page not display at all or display incorrectly. There is not a workaround for this, other than that as Firefox rises in popularity more pages are being coded to comply with accepted internet standards.
Another reason some pages will not display is that they are set to detect what browser a person is using and reject the connection if they are not using a specific browser. There are extensions which can circumvent most of these issues, but they are beyond the scope of support of AT&T.