How to get ESP8266 Wifi MAC ?

Do you mean when running Espruino on the ESP8266 itself, or when running on an Espruino board connected to the ESP8266?

If it's on the Espruino board I'm not sure there's a function, but you can add one with a simple bit of code. Something like:

  wifi.getMAC = function(callback) {
    var mac;
    at.cmdReg("AT+CIFSR\r\n", 1000, "+CIFSR", function(d) { 
      if (d.indexOf("STAMAC")>=0) mac=JSON.parse(d.slice(d.indexOf(',')+1)­); 
    }, function(d) { 
      if (d!="OK") callback("CIFSR failed: "+d); 
      else callback(null, mac); 
    });
  }

//
wifi.getMAC(function(err,mac) {
  console.log(mac);
});