In Firefox at least, images must be completely self-contained i.e. everything in one file for privacy reasons.
If you want the fonts to work you’ll have to base64 encode them and embed them as a data URLs in the SVG file i.e.
@font-face {
font-family: 'Open Sans';
src: url("data:application/x-font-ttf;base64,[base-encoded font here]");
}