Greetings all, I spent the better part of the weekend trudging through the internet looking for others with this issue. Granted, I'm still novice with Linux, I feel there must be a solution out there I'm missing.
Ubuntu MATE 20.04.1 LTS
Raspberry PI 4 8GB
128 GB Samsung U3 Micro SD
2 displays connected @ 1600x900
Ethernet
USB Keyboard/Mouse
CPU and GPU overclocked (but this was happening with the standard config as well)
Base image: ubuntu-mate-20.04.1-beta2-desktop-arm64+raspi.img.xz
On both a fresh deployment of Ubuntu Server + Desktopify as well as the main download for Raspberry Pi 64-bit, I've been unable to get Youtube to play a video in Firefox or Chromium.
I've been able to get VLC and other offline apps to work however. I've tried multiple browsers and I've looked for Codec packs.
Dropping in the stock Raspberry Pi OS (Buster) the site is working under Chromium.