Hi Joey, I think the "re-used ... serial" is related to the previously saved exception. Could you delete the exception and then save a new one? I think it would be listed here:

Open the Settings page using either:

• "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• type or paste about:preferences into the address bar and press Enter to load it

In the very tiny search box at the top of the page, type cert and Firefox should filter to the "Certificates" section.

Click the "Certificate Settings" button, click "Servers", select the old saved exception and use the Delete button to remove it. Then click OK near the bottom of the dialog.

Can you save a new exception successfully?

Hi jscher2000.

After the troubleshooting I've done, the Servers tab is now empty.

When I visit the gateway address it no longer gives me the option to create an exception or bypass the http-only.

I also deleted the saved data for the gateway address.

You don't get the error page that has the "Advanced" button? What error do you get?

Same one in the main post.

I've attached a screenshot.

The language is odd: "You are attempting to import a cert ..." But if I understand the scenario, you're just trying to open the page.

Maybe what Firefox is importing is an intermediate certificate used to sign the site certificate. If you visit the router in another browser and view the certificate, and check its signing certificate, then you could browse through Firefox's Certificate Manager Authority tab (which is organized by issuer) and see whether there is a saved certificate there for that same issuer which is not one of the built-in certificates. (I don't know of a direct way to search by issuer and serial number.)

From Edge the certificate is empty(?) (I've attached a screenshot, but not including the SHA fingerprints).

And Edge (like Firefox used to) asked to confirm I'm sure to visit.

I appreciate your patience.

Try to rename the cert9.db file (cert9OLD.db) and remove a possible previously used cert8.db file in the Firefox profile folder with Firefox closed to remove intermediate certificates and exceptions that Firefox has cached.

• https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean

If this has helped to solve the problem then you can remove the renamed cert9OLD.db file. Otherwise you can undo the rename and restore cert9.db.

You can use the button in "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Hmm, root.home. Based on a web search, it may be a signing certificate used in Huawei routers.

Does your Firefox Certificate Manager, Authorities list, have any authority listed as root.home?

It's indeed a Huawei router.

No. No such certificate. And nothing by Huawei. I tried exporting it from Edge and importing it into Firefox but nothing happened.

I also tried Firefox in troubleshooting mode, which disables any extension.

Thank you so so much cor-el. I've marked your reply as the solution. I renamed the file as you said and voila! There was no cert8.

Also thanks again jscher2000.