If you access a site over tls/ssl with Firefox and the server's certificate is not signed by a cert in Mozilla's root store you will get a page telling you the connection is untrusted.
If you expand "I Understand the Risks", you can add either a temporary or a permanent exception.
To avoid these altogether you can use the MITM extension (
https://addons.mozilla.org/en-us/firefox/addon/mitm-me/). As the name suggests, it is perfect for people who want to get MitM attacked.
-mancha