I have a problem with SVG renderring in Firefox. Whilst it works in Safari, Chrome or even in IE, it doesn\'t work in Firefox. Inspecting the element it seems that the layer
You're experiencing bug 995813.
I fixed this bug some time ago but that change is still making its way through the release process so your example will display correctly from Firefox 31 onwards which will be released on 22 July 2014.
In the meantime if you move the fill="white" from the <mask>
element to the child <use>
element this will work around the Firefox bug.