At least the spawning problem seems to be caused your layout scaling, instead of 'Scale' you should use 'letterbox scale' or similar, take a peek at layout scaling stuff at manual/soem tutorial. Seemed to solve the problem for me at least. I'd imagine the wrapping issue might be caused by this as well.
Just tried letterbox scaling, still having the enemy spawn off-screen in Firefox.
I created an event that checks for the enemy being offscreen. Basically the event looks like this:
if Enemy is onscreen, set Enemy.IsOnScreen to 1.
if Enemy.IsOnScreen = 0 (it's zero by default), then
-Every tick, set enemy position to Enemy.X + 1, Enemy.Y + 1
It doesn't seem to work, however, because when an enemy spawns off the screen it's not coming around. I have my own custom wrapping code that is supposed to bring the enemy to the other side of the screen once it exceeds WindowWidth/WindowHeight or if X/Y is less than zero, but those aren't working either.
Maybe it's just an issue with the latest version of Firefox (version 19.0.2)? This doesn't seem to have problems on Safari.