I'm using Firefox Quantum 68.0.1 on a Macbook Pro with macOS 10.14.5 with a Dell external monitor connect via HDMI using extended display. I've noticed that some of the icons in gmail disappear when I move the browser to the external display. This doesn't appear to be a hardware issue since you can see that the icons are missing in the screenshot for the external display. Moreover, if I drag the window from the internal display to the external, I can see the icons until I release the window, at which point they disappear.