+ Firefox: only CSS filter with `url()`

https://developer.mozilla.org/en-US/docs/Web/CSS/filter#Browser_compatibility
Fx Almost v31+
Fx No

https://github.com/Fyrd/caniuse/pull/609#issuecomment-48017258
revert to Almost; url() is part of spec

I spec for both CSS-HTML & SVG: http://www.w3.org/TR/filter-effects/#abstract
Merge branch 'tomByrer-patch-1'
Correct CSS filters for Firefox