I never saw it in High Sierra, but after upgrading to Mojave last week, I see it once in a while after resuming from sleep.
And maybe even some not so recent standards. Chrome has an extremely novel feature:
I can use my media keys...
...to control the playback of my media.
Why didn't anyone think of that before? :)
...or Firefox simply has some performance problems.
Touching the DOM in Firefox is a terrible bottleneck, for example
Yep. Speedometer 2.0 shows this pretty well. It's 25% faster in Chromium for me on desktop, and over 100% faster! in mobile Chrome than Firefox for Android. Mozilla have done a remarkable job of catching up on raw javascript engine performance and the speed of the renderer, but they have a ways to go yet in other areas, probably none more so than bringing recent improvements to mobile.