There are more possible causes apart from a firewall. If Firefox can't find a website then this means that a DNS lookup fails (Firefox can't resolve the domain to an IP address), so this can be a problem with the DNS server(s) that are currently used. Firefox can use special DNS servers. *https://support.mozilla.org/en-US/kb/firefox-dns-over-https *https://support.mozilla.org/en-US/kb/dns-over-https-doh-faqs This can also be caused by using an VPN or proxy. See "Firefox connection settings": *https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own. 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 https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message https://support.mozilla.org/en-US/kb/connection-untrusted-error-message [https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors?redirectlocale=en-US&redirectslug=Error+loading+web+sites Websites don't load - troubleshoot and fix error messages] http://kb.mozillazine.org/Error_loading_websites [https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean What do the security warning codes mean]