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.
MBP 2013 with integrated graphics, running Mojave and the latest stable public release of Firefox. I haven't messed with about:config.