Convert to MP3 is an add-on that let you easily convert audio or video files to mp3 file format.
Simply, drag the audio or video file in the add-on's UI (top section). Once the media is added to the UI, click on the convert button to start the conversion process. Before conversion, you can adjust some settings including audio bit rate, audio sample rate, and audio channels. Moreover, you can trim the audio at desired periods.
The core for this add-on, is lame.js
page for this add-on to get more info on how to work with this addon.