I installed the basic version of Ubuntu Desktop, this came without loads of extra software that I didn't need or want (Amazon, LibreOffice etc.) However, when I came to watch YouTube I wasn't able to due to lack of HTML5 support.
HTML5 support can be tested here: https://www.youtube.com/html5
Luckily a nice easy fix to install the missing codecs and get playback working again, in a terminal (Ctrl+Alt+t) run the following command
sudo apt install ubuntu-restricted-extras
Afterwards just restart Firefox if you still have it open and playback should work as expected