SSL & Firefox: How to make “Permanently store this exception” checkbox active

When one accesses webmail or the Plesk control panel using ones own domain name, one will see a warning for "Domain mismatch" from the SSL certificate.

This happens because we secure those URLS with our (*.his.com) SSL certificate, which is issued to match the HIS server name, but does not match your domain name.

Usually, one can just click "Advanced" and then "Add an exception" in Firefox, but sometime the "Add exception" button is greyed-out.
Here is how to make “Permanently store this exception” checkbox active

In Firefox, The procedure is:

• Go into Options, then Privacy settings
• Click History and select "remember history" in the drop down menu.
• Restart Firefix and then reload the webmail access page with the security exception choice.
• One is now able to check the box "permanently store this exception"
• Afterwards you can go back and re-select "never remember history" choice in your settings if you want to keep it this way.

Source: How to make “Permanently store this exception” checkbox active?