Go to "Tools" >> "Options" >> "Security" >> In this tab click on Exceptions, then you will see the exceptions that you added , to delete select one and click on Delete button

The above reply relates to sites you allow to install add-ons in Firefox. If you were trusting the site in some other way, could you add a little more information to your question?

For example, if you trusted the site's security certificate even though Firefox thought it might be invalid, those are listed in this dialog and can be removed there:

orange Firefox button (or Tools menu) > Options > Advanced > Certificates mini-tab > "View Certificates" button

On the Servers tab, you will see both sites that you decided to trust, and special entries for certificate authorities indicating that none of their certificates should be trusted. To spot the difference, look at the Server column. The ones with an asterisk (*) are Firefox's entries and you shouldn't remove those.

In the future, I suggest, when making an Exception, uncheck the box that adds a permanent exception. That way, when you exit Firefox, the exception is removed automatically.

Final thought: if you are having to add more than one exception in a short period of time, or exceptions for popular sites that obviously have their security act together, your issue may be solved in a different way. For example:

• Security software that intercepts and filters secure connections (e.g., ESET, BitDefender, Kaspersky) - you can have Firefox trust your software, or you can turn off this feature

• Malware that is trying to steal information from your secure sessions - obviously you want to clean this out ASAP

If you're not sure who is behind these certificate errors, post the problem address and have someone check the details in the real certificate. Then you can compare what you're getting when you open the Add Exception dialog, click the View button to check the certificate issuer and other details.

Hopefully we can get this all cleaned up for you.

Thanks, Jefferson. You covered the issue beautifully, and delightfully comprehensively.

I found and deleted the server where I granted the exception.

"In the future, I suggest, when making an Exception, uncheck the box that adds a permanent exception. That way, when you exit Firefox, the exception is removed automatically."

Aha. That is very good to know. I missed it when granting the exception today.

The exception I granted (and now deleted) today was not really an unsafe site. I know the owner. Just a glitch in their link.

Thanks again. And thanks Toddy ... I didn't specify clearly what kind of exception.