Edit: Clock is automatically set via OSX. Not a problem with other sites.
Firefox says
tails.boum.org uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. (Error code: sec_error_unknown_issuer)
Sooooo..... I need a root certificate of some sort then? See, this is what we get to contend with - I can't read this site because reasons. And it's up to me to find out what the reasons are I guess. Wait till this hits the masses when certs get revoked, expire, etc. :)
If that is the case then your browser is exhibiting correct behavior.
For me, I can see that the root CA is USERTrust (SHA-384 sig, interestingly), and the server is presenting a valid intermediate (Gandi - also using a SHA-384 signature), then the site certificate (SHA-256 sig).
There is a secondary certification path though, coming from a old SHA1 AddTrust Root (but this is also in my trust store for Firefox).
Are there tools I can use to work my way through this?
OpenSSL is a good starting point:
openssl s_client -connect tails.boum.org:443
SHA-256 Fingerprint for tails.boum.org should be:
F8:DC:67:21:96:77:46:F5:9D:77:BD:7B:87:C1:39:42:C8:4E:4B:25:97:34:AC:E2:80:24:99:35:D9:81:9C:B6
If that doesn't match the value you see in the Firefox or Chrome certificate details page, please, send as many details about the chain as you can back, I'm very interested to see what's happening here considering you're not on a corp network and seeing this and even moreso because this is the Tails site, something that might very much interest some attackers...
But the problems seem to have gone away.