playing you tube in slow motion on firefox

I am running Ubuntu 16.04 and firefox 58.0.2. I wish to play cad instructional videos in slow motion. I have read you have to use html5. But. the mozilla says my version of firefox is not compatible. Web searches bring up links from over 5 years ago and are now useless.

How do I play you tube videos in slow motion.

tj

please tell us exactly what firefox told you.
do you perchance have a (outdated) flashplugin installed?

meanwhile you can always use the link to the video to watch it with your favorite media player.
vlc, mplayer and mpv are capable of doing that.

youtube-dl -f 18 <yt_url> -o MyVid.mp4

If you want to keep the same pitch but slowed down

mplayer  -af scaletempo MyVid.mp4 -speed.7

If you want the audio and video slower

mplayer -speed .7 MyVid.mp4

mpv or other video players will do the same thing.

Look at
man youtube-dl
man mplayer