You might try deleting any previous exceptions you created.

orange Firefox button or classic Tools menu > Options > Advanced > Encryption

If you click the View Certificates button, then the Servers tab, the itacs.fda.gov site should be listed under "(Unknown)". Try deleting the certificate and seeing whether you are now allowed to add it.

For what it's worth, this is a problem with the FDA site. When sending their certificate, they are supposed to send any intermediate certificates required to complete the chain up to the trusted root certificate. IE somehow is filling in the missing link through its own magic, but Firefox requires that it be sent by the actual web server.

The itacs.fda.gov/ server doesn't send a required intermediate certificate.
If Firefox hasn't saved the "Cybertrust Public Issuing CA 1" on a previous visit to a site that has send it then you will get an error message.

You can contact the website to inform them about this issue and ask them to install the missing intermediate certificate.

You can inspect the certificate chain via a site like this:

• http://www.networking4all.com/en/support/tools/site+check/