These certificates causes your browser to display a warning (and Firefox warning is pretty dissuasive), because they aren't secure (they're vulnerable to man-in-the-middle attacks).
StartSSL "class 1" certificates, albeit free, are signed by a Certification Authority (StartCom), and display no warning in your Web-browser. (They aren't vulnerable to man-in-the-middle attacks).
but as long as typical hijacker, which is frequently feds/isp, can/might hijack you isp, he can mimic CA activity too, with help of altered browser binary update. there is no way to combat that, than enforce both IPv6 deployment/usage for any kind of mission-critical/society-critical/survival-critical intrastructure/network with enforced crypto and DNSSec too, while both isn't invulnerable, but step ahead.