FireFox 21.0 – This Connection is Untrusted on everything ever

If your system date or time is not wrong, and your problem is still not resolved, try the following. Because this problem is occurring only in your Firefox, but not IE, navigate to the secure site in IE, and determine which certificate is being used, which will be an icon somewhere around the menu bar. Then go to IE > Tools > Internet options > Content > Certificates > Trusted root certificate authorities (or maybe Intermediate certificate authorities), and find the certificate. When you find it, select Export, to export the certificate, in the default format.

Now go to Firefox > Tools > Options > Advanced > Encryption > View certifications > Authorities, and look for a certificate(s) with a similar name. If you find one, press Export, to export a backup copy of it. Now import into Firefox Authorities the certificate you exported from IE. Exit and restart Firefox. See if the "untrusted connection" problem is now fixed. If not, you could delete the imported certificate, if you wish. If problems arise, import back into Firefox Authorities the original Firefox certificate you backed up.

As soon as you finish, and everything is working OK, delete the certificates you exported, because they must be kept secure and must not be kept as files on your computer.

It's a really dangerous thing to do, because you will not be able to detect a Man-in-the-middle attack anywhere (at home, on a public wifi...)

The solution is to add the root certificate used by your entreprise proxy as a trusted authority (CA) in your browser, as described here :

https://wiki.wmtransfer.com/projects/webmoney/wiki/Installing_root_certificate_in_Mozilla_Firefox

To obtain the proxy certificate, click on the lock icon near the URL of an HTTPS page, click "View certificate", and on the windows click on your proxy's certificate (like here) then export it and proceed as explained above to import it as trusted CA.