Angular Error: Your app is not responding on the desired port

Upon running my example application I receive the following error message:

Your app is not responding on the desired port.

The port is 4200, changing it does not help. I am using Mozilla Firefox browser.
Also tried npm start after pressing run button, still receiving the same error.
Could you kindly advise how to solve this issue?

Many thanks in advance.

Addendum: I switched to Google Chrome browser - apparently, the issue does not appear there, the app runs successfully.
Still, any ideas why Firefox might be different are welcome.

Thank you.

Now the issue is also appearing in Google Chrome.
Any suggestions on what is causing this and how to circumvent it?
Many thanks in advance.