Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1) Gecko/20090616 Firefox/3.5 works for me perfectly : no temporary exception was needed I tested with a deleted cert8.db file, so that only built-in certificates would be used. The chain of certificates used by www.videobuster.de added the UTN-USERFirst=Hardware certificate (under AddTrust, serial number 26:21:1B:F5:2A:EB:51:B0:0B:FA:9F:DD:8D:36:DA:9E), while there is a similar named certificate (built-in) under The USERTRUST network, serial number 44:BE:0C:8B:50:00:24:B4:11:D3:36:2A:FE:65:0A:FD) I don't know if this is normal or not.
Oh, right. Checking with a fresh profile worked for me too. The "AddTrust External CA Root" is marked as non-trusted for me but I can only partly remember why. There was an issue with that CA at some point: http://benjamin.smedbergs.us/blog/2008-12-24/how-to-disable-the-comodo-root-certificate-in-firefox/ Ok, still the main issue is not about this particular certificate but to reproduce it, it should be enough to uncheck the trust bits.