cvp, did you resize vertically? If you resize horizontally (make it wider or narrower, its ok, but making the window longer/shorter leaves the sprite at completely random locations, some times off the screen at the top, sometimes half way down the screen. It only works if you make it longer/shorter really slowly and carefully. The same code for X (i.e. for horizontal movement) is fine, its just vertically that its hugely buggy.
In FF, I sometimes get a blank black screen also.
I just tried it in FF, and its just as buggy. Try making the browser longer quickly in jumps.
This bug used to be in Anchor, many versions ago (99?) which is why I dont use it any more. Until now, setting the Y manually has worked instead.
If i change the project to set Y every tick, its fine, but doing operations every tick like this will affect performance.
I suspect that its the Browser On resized which is the problem - its missing re-size events for what ever reason.