Masks (Obscured and Hidden) are not working properly for players who use Firefox.
They just see the whole map. Exception: If they scroll until all tokens are barely outside the visible field it's working propery, the map gets darkened and you can only see the lighting area around the token.
(GM's view is not affected.)
I played around a bit and found interesting behaviour: Everything works as intended if the player adds a token to the map. But the mask/fog disappears the moment the host adds a token.