Hi petiba, did your Firefox perform a Refresh during or just after the last update? The tell-tale sign is an "Old Firefox Data" folder on your Mac desktop. If you see that, we could suggest how to use it to restore your previous certificate store (cert8.db file) which should contain the missing certificate.

Hi jscher2000, Thanks for the quick reply. No, it did not perform a Refresh and there is no "Old Firefox data" on the Desktop. I have a back-up of the certificate, and I can import it successfully, but it is not there after importing.

Hi petiba, I'm not sure what you mean by "it is not there after importing." Just to review, you are using this method:

"3-bar" menu button (or Firefox menu) > Preferences

Then in the left column click Advanced, and on the right side, select the "Certificates" mini-tab. Then click View Certificates and choose the "Your Certificates" tab (as shown in the attached screen shot). After you import to this list, it should show up here.

Hi jscher2000, Thanks for your reply. Sorry for disappearing, but had lots of work. Nonetheless, I made a short screen capture video of my problem. I had the certificate installed, it was working properly, but at some point around two-three weeks ago - and this is why I contribute it to the Firefox 50 version upgrade - it stopped working, and I realized, that the certificate is not there anymore. In addition, when I try to import it again, it just does not show up, as you can see in the video. (I imported the certificate into my mac keychain and I use Safari with it, so it is an existing and valid certificate, and I also know the password - just to exclude some potential errors.) Please have a look and thanks for your help again. Regards, PetiBa

Try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file.

Firefox will automatically store intermediate certificates that servers send in the Certificate Manager for future use.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Hi cor-el, I could import the certificate now. Thanks for your help and kind regards, PetiBa