HOW TO DELETE ALL CERTIFICATES in FIREFOX
(Rename) or delete the following in Firefox Profile Folder
1. secmod.db (secmod.db.old) and
2. cert8.db (cert8.db.old)
3. cert_override.txt (cert_override.txt.old) --> If you have a file cert_override.txt
2. cert8.db (cert8.db.old)
3. cert_override.txt (cert_override.txt.old) --> If you have a file cert_override.txt
Profile folder on MAC OS X -Mountain Lion is usr>library>application support > firefox
for WIN OS follow the link https://support.comodo.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1254
Happy Surfing