Why am I getting sec_error_unknown_issuer in Firefox?

We are getting a sec_error_unknown_issuer on ONE machine in the building using Firefox trying to get to https://www.dmv.virginia.gov/dmvnet/extra/securid/whatcanido.asp.  This machine is using Firefox 3.0.9.  Other machines using 3.0.9 do not have this trouble.

Where to begin?  What's going on?
Who is Participating?
If you go to this address: http://www.sslshopper.com/ssl-checker.html
and input "www.dmv.virginia.gov", then let it check the SSL, you see that the "certificate will not be trusted in all web browsers".

This is true, because I tested it in Opera and it works fine. Why only a few Firefox machines at your place are affected, I cannot tell, my Firefox didn't like the certificate either. I assume it could have something to do with already having the certificate chain installed from earlier, versus going there first time.
Maybe the server admin reinstalled/renewed the certificate and did not do it correctly?

Anyway, here's the remedy:
Go to Entrust's root certificate download page: https://www.entrust.net/downloads/root_index.cfm
on the Download page, scroll down to the last certificate (Entrust L1B Chain Certificate), right-click it and select "Save target as...".

In Firefox, open Tools > Options > Advanced > Encryption > Show Certificates
Select the tab "Certification Authorities" (last but one), click "Import" and point it to the .cer file you just downloaded from Entrust. Click 'Ok' a couple of times and restart Firefox.

That solved the issue for me.
mrcoulsonAuthor Commented:
Okay.  I just installed 3.0.9 on another machine and this is happening there, too.  Hmmm.  But it's still only some machines.

When importing the certificate, make sure to place a checkmark in for the kind of certification you wish (websites for sure, maybe email, too?)
mrcoulsonAuthor Commented:
Bingo!  And thanks for the resource in sslshopper.com.

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