When I'm trying to resize the "Welcome to Firefox" window on a Retina iMac on 10.12.6, the wavy background animation completely freezes. Firefox then takes more

When I'm trying to resize the "Welcome to Firefox" window on a Retina iMac on 10.12.6, the wavy background animation completely freezes. Firefox then takes more than a second to update the window content and restart the animation. That particular welcome screen seems like a really poor choice to demonstrate the supposedly much-improved rendering engine on my machine.