Not able to secure website on Firefox even after installing SSL certificate

I am using the below code to install SSL certificate on my website:

const express = require('express');
const path = require('path');
const app = express();
const fs= require('fs');
let privateKey = fs.readFileSync('certificate/x.key', 'utf8');
let ca = fs.readFileSync('certificate/x.crt', 'utf8');
let certificate = fs.readFileSync('certificate/x.crt', 'utf8');
let credentials = { key: privateKey, cert: certificate, ca: ca };
const http = require('http');
const https = require('https');

app.use(express.static(path.join(__dirname, 'build')));

app.get('/*', function(req, res) {
  res.sendFile(path.join(__dirname, 'build', 'index.html'));
});

let httpsServer = https.createServer(credentials, app);

httpsServer.listen(443);

This is a react app and I am serving it via Node.js.

When I open the website on Chrome or Microsoft Edge, it shows the connection as secure, encrypted and valid but when I open it on firefox, it shows that the connection is not secure.

What could be the problem?

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