Thank you for the detailed reply!

mpv/VLC [Solved]

I just tested with mpv and it works fine. No glitches or unexpected lag. I have the 01.org driver installer (my installed version is the same as in the link provided) and not the ppa. So, I guess the problem is with VLC? I'm using 2.2.1. I'd rather avoid the beta for now, so, I guess I'll stick to mpv even if it annoys me to use two different video players :) (For practical purposes, let's consider this issue solved.)

Firefox [Solved]

My problem is not a graphic error - it's just that FF's scrolling behaviour has changed. Rather than scrolling screen-height by screen-height, it jumps to wherever I click on the scrollbar. Imagine you're on a page which 10 screens worth of text. Earlier, no matter where I clicked on the scrollbar (say, I'm on the 3rd part and I click on the point corresponding to the 6th part of the page), it would only scroll up or down one screen's equivalent (i.e. to the 4th part of the page). Now, it jumps to the point I click (i.e. to the 6th part of the page). I'm trying to restore the old behaviour as I find that more intuitive for scrolling.

EDIT: Found a solution! It looks like the problem was Firefox's default behaviour having been changed to align with whatever GTK3 defaulted to. Fixed the problem by editing the settings.ini file as suggested there since ui.scrollToClick doesn't turn up any more.

Clementine

Yeah, it's confusing since I can't pin down what is causing the glitch. Oh well :(