Hello,
kindly please help me if you can ...
I installed an Ubuntu-MATE on my Raspberry PI 4B 8GB and I am using it as desktop every day. I did it on base of many advices on internet and I can confirm, that the Ubuntu MATE is really comfortable, fast and good looking OS. The installation was done from image Ubuntu Server 20.04 and then using Desktopify.
The OS was running fine couple of weeks but last days I am experiencing more issues. I am not sure what happened:
Issue 1 - Video playback:
One day (probably after Firefox update) the video quality in Firefox bacame poor. There are strange color dots arround moving objects. Doesn't matter which video quality I choose. The issue is only in Firefox (youtube, facebook). The issue does not occur in Chromium nor locally stored video files. I tried to uninstall FF (apt-get remove --auto-remove --purge firefox) removed all firefox files which I found in /etc and my home and then installed FF again from Software Boutique. I also enabled "layers.acceleration.force-enabled" . Nothing mentioned helped.
Issue 2 - HDMI sound:
Suddenly the sound over HDMI stopped working. If I switch to Headphones over PI's jack, that works fine. But back to HDMI not. I noticed in Sound settings, that the outupt shows "Analog" and I would expect "Digital". Im not sure if this is the issue, but as I remember, this was there also before when it worked. If I boot an Raspi OS from SD Card, the HDMI sound works fine, so this should not be a hardware problem.
Issue 3 - HDMI reset:
On some page I found an advice to put "dtoverlay=vc4-kms-v3d-pi4" into "config.txt". Well I tried it (at the beginning after installation) and I had a feeling that the video playback was better (less dropped frames). However every time I moved the mouse over played video, the monitor did something like "reset" = switched off and on shortly. I tried different HDMI monitors and I got the same result. So I resigned and didn't use that option.
A strange NOTE:
I did just a try / an attempt and I put "dtoverlay=vc4-kms-v3d-pi4" into "config.txt" these days again and I realized, that it has no influence on Issue 1, but it has influence on Issue 2. The HDMI sound works fine with that settings and Sound settings shows "Digital" output. However I can not keep this settings because of issue 3.
Of course, I tried to google the solution but without success 
I hope and believe you understand my concern. I am looking forward and thank you for any advice.