I picked up some malware recently and one of the many annoyances it caused was problems with Firefox's security settings.  When visiting secure (https:) web sites Firefox would say "the connection is untrusted." When I said I wanted to trust the site and add an exception for it I would get a "sec_error_untrusted_issuer" error.  When I got past the errors and added the exception the site still wouldn't work properly -- I would only see a plain text version of the secure page without images, buttons, or formatting.

After many days of grief I finally found the answer in Mozilla's bug database: https://bugzilla.mozilla.org/show_bug.cgi?id=583191 (and yes, I got the same error when visiting Mozilla's site).  I first tried the recommended troubleshooting steps without success.  I then tried the following approach, recommended by Matthias Versen.  (I did this with Firefox 3.6 and Windows 7.)

  1. Type the phrase about:support as a URL in the Firefox address bar.
  2. Click the button next to Profile Directory labeled "Open Containing Folder". That will launch Windows Explorer.
  3. Close Firefox.
  4. Locate the file cert8.db in the Windows Explorer pane you opened in step 2 and move the file to a different location (like your desktop) or rename it.
  5. Start Firefox again.

All I can say is that it worked for me.  Maybe it will help you.