Today I read that the browser extension "HTTPS Everywhere" is now deprecated (https://www.golem.de/news/eff-https-everywhere-wird-nicht-mehr-gebraucht-2109-159883.html).
I followed the advise from the EFF and enabled the 'HTTPS-Only Mode' in Firefox (https://www.eff.org/deeplinks/2021/09/https-actually-everywhere), afterwards I removed the extension.
Now everytime you visit a website, which doesn't offer an encrypted version you'll get notified and actively need to acknowledge this, before you get directed to the unencrypted connection.