Configuring your firewall on Mac OS X 10.5
By default Signal will be added to the application firewall automatically and no configuration is needed. However, if the firewall has been set to “Allow only essential services” this will block all applications from receiving incoming connections, including Signal.
To configure the firewall:
- Open System Preferences, click Security, then click Firewall.
- Ensure that the firewall is set to either "Set access for specific services and applications" or "Allow all incoming connections".
You can read more about the available application firewall options and what they mean at Apple’s support site.