The problem is server side and technically the certificate.
OldFatGuy: Same thing happens, although the screen looks a little different in Firefox. (See attached)
Next time expand the
Technical Details.
Someone likely forgot to include
mg4.mail.yahoo.com in the sea of all the other valid
Subject Alternate Names, or it's technically the wrong cert for that server. It is something yahoo needs to fix, but it
is a valid yahoo cert. Just not for that specific domain.