ASP.NET Core Self Signed Certificate in Firefox not working

Firefox does not use the Windows Certificate Store, so you'll need to add the certificate as an exception to Firefox manually.

Start the development server from Visual Studio; in Firefox, goto top-right hamburger icon -> Options -> Privacy & Security -> Certificates: View Certificates... -> Add Exception... and enter the url of your local development server, e.g "https://localhost:4000". Click "Get Certificate", check "Permanently store this exception" and "Confirm Security Exception".


The Firefox browser uses its own certificate store, and therefore doesn't trust the IIS Express or Kestrel developer certificates.

There are two approaches to trusting the HTTPS certificate with Firefox, create a policy file or configure with the FireFox browser. Configuring with the browser creates the policy file, so the two approaches are equivalent. See https://docs.microsoft.com/en-us/aspnet/core/security/enforcing-ssl#trust-ff on the official and maintained docs.