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How to get Firefox to accept a Self Signed Certificate on a private network?

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I am trying to test a web server on a private embedded device using Firefox as the web client on a PC on the same LAN. There is no DNS service and no access to the internet.

We have created a self-signed certificate to expire in 1 years time (2 Oct 2021).

When we connect to https://IP-Address-EmbeddedDevice from a Windows PC (win 10) and using Firefox to browser to we get the yellow warning triangle.

Clicking on the warning triangle tells us check the certificate and when viewing the certificate (the Website:, Owner: <name>, Verified by <our company name>, Expires on: <Saturday, 2 October 2021>.

If I click on View Certificate, firefox will open a new tab with a URL: about:certificate? but will not display anything but a blank page.

I have tried "adding the certificate to the Personal store" in Windows 10 and the import of the certificate was successful. Yet when I open the URL: I still get the yellow warning about the SSL certificate.

Could you advise on how we can self-certify our webserver such that the browser on client PCs do not report this self-certificate as a warning?