I found this link:
Since the -moz-linear-gradient(19% 75% 90deg, #FA8246, #BB5422)
syntax is only for Firefox 3.5 (i am not sure about this) and older, we think it makes sense to teach the newer (and simpler) -moz-linear-gradient(top, #FA8246, #BB5422)
syntax instead.
Anyway, you should adopt what is in the documentation now.
Something like this works, like replacing the position (left, right):
background-image: -moz-linear-gradient(50% 0deg, white, #BB5422)
that is equal to:
background-image: -moz-linear-gradient(0deg 50%, white, #BB5422)