YouTube™ to MP3 is a simple and powerful add-on that let you convert and download the streaming YouTube video directly to mp3 file.
Simply click on the in-page button (see the screenshot) to download the mp3 file. You can also change the default mp3 quality from the add-on options page. There are five qualities to choose from and the default quality is 128kbps. Please note that, you can convert multiple YouTube videos to mp3 as well.
Closing the active tab or pressing on the delete button will stop the active conversion process.
Note: this add-on is forked from "Audio to MP3" add-on written by Sevina for Firefox (https://addons.mozilla.org/en-US/firefox/addon/audio-to-mp3/). It is specifically tailored to work on YouTube pages only. The "MP3" button is placed below the video in YouTube page.