Firebase serve error: Port 5000 is not open. Could not start functions emulator
For Mac/Linux, use the Terminal/Shell to find the Process ID (PID), then kill the process.
sudo lsof -i :5000
sudo kill -9 PID
In MacOS Monterey, port 5000 may be claimed by a new "AirPlay Receiver". This can be disabled in Settings -> Sharing:
Screenshot of settings panel for disabling AirPlay Receiver
Disabling the AirPlay Receiver (if you do not need it) frees up port 5000.