I'll bump this thread and may as well describe my problem before trying to fix it with what may not be the solution!
Ever since I upgraded to 20.04, I switched to Mate as Gnome flashback no longer worked properly. Not sure if it was related to mate specifically, but youtube has always had bizarre issues with something, but not exactly screen tearing. Below is an image to show what I mean.
The problem isn't so noticeable during motion in a busy scene that much, but the 2 areas I've highlighted are an area that suffer most often. Basically, I get what look like very faint filters in these boxes I have highlighted making the image that bit darker. If the video has a section with that part all one colour with a bit of motion, the box at the top left is very obvious. As is the much larger box on the right. If paused, these lines/boxes go. They also go briefly when taking a screenshot! They can vary in size and position, but mostly occur at the size and position i have shown. They are less visible and in a different location when in full screen.
An important note to make - I use firefox and this never happens in chrome on youtube. I also never actually get tearing in firefox on youtube so I don't know what this problem is.
On the topic of tearing, Ubuntu IMO seems to have got far worse with it. Back on 14 and 16.04, I used a DVB PCTV stick and the interlaced freeview footage actually worked well with VLC. Ever since 18.04, most video players are terrible at handling footage that isn't de interlaced. I've recently got my tv stick working again with kaffeine, but it looks terrible whenever there is any motion. News text moving horizontally is almost unreadable when playing the video.
Something else that I have a problem with most media players is as this screenshot shows:
This is playing a recorded video on my PC using MPV, but it is the same on almost all programs. The area I have highlighted in red is what seems to tear almost every time the picture has horizontal movement. It is always in roughly the area between the two red lines. So a chunky bar that looks like it is slightly separated from the rest of the image during movement. I assume this is tearing? VLC does this too since the upgrade to 18.04 and now about the only player that doesn't is Mplayer. This is an incredibly dated program that I use through SMplayer. It has it's disadvantages, but it is about the only one i can watch without having this bar appear during horizontal motion.
Which of the mentioned methods may help sort these issues out? For years I don't seem to have been able to watch videos without some form of tearing or interlace issues. But has got worse with 20.04.