It seems like this is a known bug : Issue #28 Notary fails for sites relying on SNI.
The description of it is :
Convergence will currently give a certificate error for sites using
SNI to provide appropriate certificates for virtual hosts; it looks
like this will magically fix itself once Twisted supports SNI.
Understanding where the problem lies takes some doing, tracking down all the links
to all the bug tickets this bug report is linking to. Apparently this requires
SNI support in PyOpenSS, meaning on the server side, so that github.com must also
update its website's software.
SNI is short for Server Name Indication, defined by wikipedia as :
a feature that extends the SSL and TLS protocols to indicate what
hostname the client is attempting to connect to at the start of the
handshaking process. By doing so it allows a server to present
multiple certificates on the same IP address and port number and hence
allows multiple secure (HTTPS) websites to be served off the same IP
address without requiring all those sites to use the same certificate.
Unfortunately to make use of SNI practical it is necessary that the
vast majority of users are using web browsers that support it. Users
whose browsers do not support SNI will be presented with a default
certificate and hence are likely to receive certificate warnings. As
of 2012 there are still many users of browsers that do not support
You should first ensure that you are using the latest version of Firefox.
If this is not enough, you will need to wait for all the software on both server sites
and client software to correctly support SNI.