It works if you use -moz-animation: fly 0.2s steps(4) 10;
. It looks like firefox is having some issues with the way you have shorthanded the animation properties.
I'm not sure what the other 0 was supposed to be for, but it doesn't look like it's needed unless I am missing something...
Edit: I verified in Chrome that the extra 0 isn't needed either:
.follow-button:hover {
-webkit-animation: fly 0.2s steps(4) 10;
-moz-animation: fly 0.2s steps(4) 10;
animation: fly 0.2s steps(4) 10;
}