Firefox 35+ seems to support the native filter property values, possibly remove the SVG filter at all: http://caniuse.com/#search=filter
}
.grayscale {
- filter: url(../images/grayscale-filter.svg#grayscale);
+ //filter: url(../images/grayscale-filter.svg#grayscale);
+ // Firefox does not allow SVG filters to be retrieved from a different domain, embed it directly (Firefox 35+ seems to support the native property!)
+ filter: url(data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%3E%3Cfilter%20id%3D%22grayscale%22%3E%3CfeColorMatrix%20type%3D%22matrix%22%20values%3D%220.3333%200.3333%200.3333%200%200%200.3333%200.3333%200.3333%200%200%200.3333%200.3333%200.3333%200%200%200%200%200%201%200%22%2F%3E%3C%2Ffilter%3E%3C%2Fsvg%3E#grayscale);
filter: gray;
-webkit-filter: grayscale(1);
}