Serve Ogg media with the correct MIME type
*.ogg and *.ogv files containing video (possibly with an audio track as well, of course), should be served with the video/ogg MIME type. *.oga and *.ogg files containing only audio should be served with the audio/ogg MIME type.
If you don't know whether the Ogg file contains audio or video, you can serve it with the MIME type application/ogg, and Gecko will treat it as a video file.
Most servers don't by default serve Ogg media with the correct MIME types, so you'll likely need to add the appropriate configuration for this.
For Apache, you can add the following to your configuration:
AddType audio/ogg .oga AddType video/ogg .ogv AddType application/ogg .ogg