In recent Firefox (e.g., 34.0.5), a URL like https://lists.freebsd.org/pipermail/freebsd-bugs/2015-January/404.html gives an ugly page without CSS. This is because Firefox refuses to load the style sheet from http://www.freebsd.org/ into an https page.
A possible solution is to use scheme-relative URLs like //www.FreeBSD.org/layout/css/fixed.css. Patching this in using developer tools makes the 404 page look normal.
Appears to have been fixed using a very simple 404 page that does not need CSS.