if you want to show youtube videos on raspberry pi you better do not go with a web browser but with the omxplayer.
this is a preinstalled videoplayer which is specially made for raspberry pis and works perfectly.
more info on the omxplayer can be found here
to show youtube videos you need the help of youtube-dl. because the package you get via apt-get is not the last version you better install via wget
sudo wget https://yt-dl.org/latest/youtube-dl -O /usr/local/bin/youtube-dl
sudo chmod a+x /usr/local/bin/youtube-dl
to play a video go to the terminal and type
omxplayer `youtube-dl -g [yotubeurl]`