Firefox bug: certification loop
This issue has been corrected with the version 15 (released in July 2012).
Firefox and by-products may encounter some loop issues during SSL certificates verification. It happens when Firefox memorizes 2 intermediate certificates that potentially loop, the algorithm not being able to find the right path to the root.
TROUBLESHOOTING: update the browser (if you have a version older than the 15)
TBS X509 and Comodo products are affected by this issue: the browser being able to memorize 2 intermediates generating a loop. Mozilla documented that bug.
When it happens, an experienced user can delete the 'personal security' kind of certificate in Firefox certificates' manager.