[Firefox] TypeError: svg is undefined TileLayer.GeoJSON.js:123
[Firefox] TypeError: svg is undefined TileLayer.GeoJSON.js:123

SVG root element is apparently not correctly created in Firefox.
Add preventive check and then create element if missing.

Livefyre/leaflet-tilelayer-geojson,glenrobertson/leaflet-tilelayer-geojson,kozer/leaflet-tilelayer-geojson,jgimbel/leaflet-tilelayer-geojson