Over the last few versions of macOS upgrades, the app-based egress control has been strengthened. The result is that non-Safari browsers will trigger a prompt like this:
If “Don’t Allow” is selected, it makes MyTools inaccessible as it is your local network that is serving up your MyTools service. You will receive an error message of:
ERR_ADDRESS_UNREACHABLE
The solution
To fix this, navigate to the Apple Menu → System Settings → Privacy & Security → Local Network and enable your browser’s access like this: