Firefox runs the same as Safari/Chrome, i.e. bad. It seems like apps need to be updated to support ProMotion. e.g Facebook Messenger (and WhatsApp) do
not run with ProMotion 120hz, but Messages does. Scrolling is so clearly so much smoother on Messages than FB Messenger/WhatsApp.
What's weird is that Firefox
is reporting higher framerates on the Blurbuster test.
Blur Busters UFO Motion Tests with ghosting test, 30fps vs 60fps vs 120hz vs 144hz vs 240hz, PWM test, motion blur test, judder test, benchmarks, and more.
www.testufo.com
But in actual usage it runs the same as Safari...