Hmm, strange. It should have 3 slashes indeed. And you did make sure the font is actually copied into /usr/share/fonts/ttf/my_font.ttf?
Our userStyle.css does the following, which works fine:
Code:
@font-face { font-family: sans-serif; src: url(res:///fonts/ttf/DejaVuSans.ttf)}
You may need to add your custom font-face definition into our userStyle.css?