I am fairly certain I’ve seen this bug, but on macOS using a MBP’s integrated graphics. I opted into WebRender a while ago via about:config, so I had figured it was just a random beta issue. Will have to see if I can capture it next time it happens!
Me too. I've seen it happen on more than a couple of occasions while visiting Trello.com and YouTube.com. Firefox isn't my primary browser, partly for this reason.
MBP 2013 with integrated graphics, running Mojave and the latest stable public release of Firefox. I haven't messed with about:config.