Why is JIT not enabled on devices like Espruino WiFi even if they are on the latest firmware?

Great - glad it helped!

When you used break and continue, did you not get an error? For example if I do:

function j() { "jit";
   for (var i=0;i<10;i++) {
     print("Hi",i);
     continue;
   }
}

It says:

JIT SyntaxError: Got continue expected EOF
 at line 1 col 42
function j(){"jit";for(var i=0;i<10;i++){print("Hi",i);continue;}}