Oh, i quite like the idea of the game, however I do think that the jumping speed requires some balancing, it might be a little too slow. Are the aliens procedurally generated? Because there was one occassion where three aliens showed up in a line, leaving me nowhere to go as well. Great job though! It was enjoyable to play!
If you used the Unity physics built in engine and then used code to modify it, especially void Update() it will function differently on each machine (at least in WEBGL) that runs it (i had the same problem originally in my game). Programming your own gravity or using void FixedUpdate() might fix the different speed issue.