I used this to generate .crt and .key files:

openssl req -x509 -newkey rsa:4096 -sha256 -days 3650 -nodes  -keyout example.key -out example.crt -subj "/CN=example.com"  -addext "subjectAltName=DNS:example.com,DNS:www.example.net,IP:127.0.0.1"

I have this node.js code:

var https = require('https');
var fs = require('fs');
var privateKey = fs.readFileSync('sslcert/example.key', 'utf8');
var certificate = fs.readFileSync('sslcert/example.crt', 'utf8');
var credentials = { key: privateKey, cert: certificate };
var express = require('express');
var app = express();
httpsServer.listen(8443);

I could use Postman and get the response using the 8443 port. When I use the browser, I get this error:

127.0.0.1:8443 uses an invalid security certificate.
The certificate does not come from a trusted source.

I installed the .crt file on my Windows machine. I also tried to import the .crt file to Firefox. When I try to import it under "Your certificates" I get this error:

This personal certificate can’t be installed because you do not own the corresponding private key which was created when the certificate was requested.

When I import it to "Authorities" it works. Why? My self signed certificate acts as a CA?