Just saw this in mozillazine - a list of sites that detect if you are using Mozilla, and block your site if you are.
Some of the error messages are hilarious. For example, one site says
It then doesn't allow access to the site, and instead provides download links for IE or Netscape (which is based on Mozilla anyway, of course).To protect you from certain usability and/or security issues we currently do not support Opera, Mozilla, earlier versions of Netscape or IE. If your preferred browser is not supported, we recommend that you consider co-installing a supported version
What kind of "usability issues" are they referring to? Surely being blocked from viewing the site is quite a serious "usability" issue!
Another site says,
To view the full Kingfisher plc site, please install a new-generation browser, such as Internet Explorer 5, or Netscape 4.x.