I was working on a computer with Firefox 20.01, and the browser would not display any SSL secured websites - not google, not facebook, not Citibank, not anything. Chrome and Internet Explorer had no trouble with these sites.
What I found was a piece of software called Sendori that was installed the previous day that had somehow altered the SSL cert information for each site. Example is below for the cert that was showing for Google.
I tried uninstalling Firefox, opening a new profile, deleting cert8.db and all sorts of things. But it was this Sendori program altering certificates that was my problem. I could see that Sendori was listed as the certificate issuer and the "valid from" date was set today for all SSL sites.
I was able to uninstall Sendori from add/remove programs.