gecko-dev/layout/style/crashtests/1161366-1.html

<script>
var f = new FontFace("x", "url(x.ttf)", { unicodeRange: "U+0041" });
f.load();
document.fonts.add(f);
f = new FontFace("x", "url(x.ttf)", { unicodeRange: "U+0042" });
f.load();
</script>