This is a really weird problem that I can’t seem to track down further. Perhaps a creative person could suggest some test ideas. Here are the facts:

• Firefox “Unable to connect” to my LAN server (a router) at 192.168.0.2 port 80.
• Network error is specifically “NS_CONNECTION_REFUSED”.
• Wireshark on a Raspberry Pi placed between the laptop and server shows no packets exchanged trying to connect. Any packet containing 192.168.0.2, any port.
• Chrome and Safari work just fine on the same machine. I can see the packets in Wireshark. This validates my test setup works.
• Curl works, loads the web page. I can see the packets.
• I have reinstalled, refreshed, removed all extensions, cleared all history and cookies in Firefox and still cannot load the page.
• Firefox in Safe Mode cannot load the page.
• Disabled DNS over HTTPS, made sure No Proxy is selected in network settings. Still cannot load the page.
• Disabled IPv6 in Firefox with about:config setting. Still fails.
• I have no security software installed of any kind on this Mac. No antivirus or firewall except the default OS one.
• Turned off Mac built-in Firewall. Still unable to connect.

Why is Firefox apparently refusing to connect to my server? Other LAN IP addresses work fine, even local ones. It specifically hates this one.