I noticed one of our sites was issuing an SSL Certificate error in Firefox with the error message “sec_error_unknown_issuer”. IE, Chrome and Safari were all good. (Who uses FF anymore? I stopped using Firefox when Chrome came out with developer tools.)
Anyway, it turns out that when you are issued your SSL cert you are also issued an intermediate certificate. My webhost had that box listed as optional and I didn’t have the intermediate cert filled in.
Once I entered the intermediate certificate the errors in Firefox went away. Took a few minutes to refresh.
I hope this helps anyone in the same situation.