This can happen if you have an incorrect user agent string (identifies the version of Firefox). Some add-ons modify the user agent string causing it to show an old version of Firefox even if you have the latest version installed.

Try resetting the user agent string so that it will correctly identify the version of Firefox that you have installed, for details of how to do that see https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser

This message that you're seeing, is that even coming from Firefox itself? It sounds more like you're talking about that balloon that appears in Windows XP about the fact that you either don't have an antivirus solution or a firewall. Firefox is neither, since it's a web browser.

Could you clarify what that error message says and where/when it pops up?