Update to latest Fira fonts for Bug 1068398

The only place we're using these fonts on mozilla.org so far is in [australis-ui-tour.less](https://github.com/mozilla/bedrock/blob/0a468e4b2797ab1bfc63c3726ce023cb95e94c42/media/css/firefox/australis/australis-ui-tour.less#L9).