TLS is the protocol invoked under the covers when viewing secure websites (those loaded with HTTPS rather than HTTP). There are multiple versions of the TLS protocol, and the most recent version, 1.2, is the most secure. Last time, I discussed tweaking Firefox so that it only supports TLS version 1.2 and not the older versions (1.0 and 1.1) of the protocol.
But that begs the question: what happens when a security-reinforced copy of Firefox encounters a website that does not support TLS 1.2? The answer is shown below.