Harry Twyford

Part 1 actually fixes the bug as filed. Firefox only lets Mozilla-signed extensions use context-fill. Add-ons declared they are created by Mozilla [by appending @mozilla.org](https://searchfox.org/mozilla-central/rev/3151f97de27730793c2e298716df760999423f26/layout/svg/SVGContextPaint.cpp#52-56,73-74) to their ID. The rest...