The Skip Cert Error Firefox extension enables skipping the SSL/TLS certificate error page, for specific configurable conditions, like self-signed cert or unknown issuer, by adding a temporary exception.
Here is a little more information about the add-on from the developers:
After letting Firefox show it’s built-in certificate error page, SkipCertError will add a temporary exception, and notify you unobtrusively. There are very few self-explanatory options, which include the conditions of the error bypass. For now, few are supported: self-signed, issuer unknown, issuer untrusted.
Want to learn more about this tool you can add to Firefox? You can pick up the Skip Cert Error add-on for Firefox from the Firefox Add-ons website.