[macOS 15.5] Firefox can't access URLs in same subnet despite 'Local Network' permission

Hi,

I have a very strange problem with Firefox on macOS Sequoia 15.5. I get a NS_ERROR_CONNECTION_REFUSED when trying to access URLs where the DNS resolves to the same subnet (e.g. macOS computer has IP 10.10.10.10 and website has IP 10.10.10.20).

I am aware of the new macOS privacy setting to restrict "Local Network" access, however, Firefox has this permission turned on in System Settings.

Things I have checked/tried that didn't fix this issue:

• Restarted Firefox, macOS and turned off/back on the privacy setting in System Settings
• Uninstalled and re-installed Firefox, including switching to the ESR version and back
• Started Firefox with a brand new profile
• Ensure network connection/proxy settings (None) in Firefox are correct
• Used the "DNS Lookup" tool under about:networking#dnslookuptool - IP resolves correctly

Firefox is able to load other websites just fine. The internal websites load just fine in other browsers (Chrome, Safari).

Any ideas on what could be going on or how else I can try and debug this?

• Firefox version 138.0.3
• macOS Sequoia 15.5 (24F74)