Unfortunately, it doesn't work for me.

works as should, but disables 'space' button on youtube streams chat

It does prevent YouTube videos from auto-playing (thank you!) but the videos appear to be buffering instead of a nice ▶ click to play like it used to be before Firefox 63.0.1 (about:config ⇒ media.autoplay=f) - don't know why that doesn't work anymore.