Hello @gordon. Thank you for chiming in. It's not easy to predict when this happens... usually on longer webpages. e.g: www.bbc.co.uk but it has also happened on this Mozilla Firefox thread page where I posted a question about the problem. And that page is not very long at all.
Each time I hover the cursor over the "slider" and it's when I left click down on it then it will sometimes just fly off down the page.... or else it won't happen until I have started dragging down to scroll down then it starts skipping and jumping down the screen in excess of what I expect from my dragging.
Today I installed Manjaro/KDE in Virtualbox (manjaro-kde-21.0.2-210419-linux510.iso)
I don't have this problem with the vertical scroll bar skipping down the page in Firefox running in Manjaro. I also installed Brave in the same OS and couldn't reproduce the problem with that browser either. So to correct myself from above, I don't see this problem in Brave.
I think it might be the combination of Firefox in Ubuntu mate. Others have reported this issue on the Mozilla forums but not sure if they are all using UMate. I considered it might be my trackpad but I installed the same Firefox version in UMate and Manjaro/KDE and I don't have the problem in Manjaro/KDE.
I hope we can get to the bottom of this because I was all set to have UMate as my daily driver OS. The menus and URL bar font size are too small in Brave and Chrome in UMate and by the way those browsers don't have that problem in Manjaro either...
Flex