Media Converter is a complete cross-platform solution to convert audio and video files built on top of the FFmpeg command-line tool.
Download Links:
- Firefox: https://addons.mozilla.org/firefox/addon/media-conversion-tool/
- Chrome: https://chrome.google.com/webstore/detail/media-converter-and-muxer/ocnfecjfebnllnapjjoncgjnnkfmobjc
- Opera: https://addons.opera.com/en/extensions/details/media-converter-and-muxer/
FAQs page: http://add0n.com/media-converter.html