I've run into a bit of an issue. When I preview things in Codepen using FF everything looks ok but if I try exporting the codepen and then preview the exact same code in FF the mask doesn't work.
This FF problem happens with this codepen : http://codepen.io/MAW/pen/AafmD
However the problem doesn't happen with this one: http://codepen.io/anon/pen/JYVdZe
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I've been trying to learn clips/masks using svg and all was going ok but then I tried it out in FF and nothing worked. That's why I thought I'd try out these codepens as they previewed in FF just fine from within Codepen.