There are two extensions in Firefox neither of which blocks the video. Music Mode for YouTube™ is the only extension that blocks the video entirely until now. These two extensions are Youtube Audio (https://addons.mozilla.org/en-US/firefox/addon/youtube-audio...) and Audio Only for YouTube™ (https://addons.mozilla.org/en-US/firefox/addon/audio-only-fo...). The second one is better and the one that recommend. Both have bugs and they do not block the video but they are better from nothing. I plan to create a version of Music Mode for YouTube™ for Firefox but, you know, it is difficult to spend a lot of time for things that you earn nothing.