You can check the connection settings. *Settings -> General -> Network: Connection -> Settings If you do not need to use a proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. See "Firefox connection settings": *https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors *https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can *https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites You can try to set network.http.http3.enabled = false on the <b>about:config</b> page. *https://support.mozilla.org/en-US/kb/about-config-editor-firefox You can check the <b>about:serviceworkers</b> page and if there is a service worker registered for this website, unregister that worker.