The "paint flashing" tool in Firefox seems to have vanished.
Both the docs and this answer are outdated. I'm running Firefox 96.0.2 (and Firefox Developer Edition 97.0b7) on a mac, and neither of them appear to have this feature. It's not under "Available Toolbox Buttons" in settings anymore.
Is it hidden somewhere new? Or was it removed since Firefox moved to using WebRender?
You're right. The reason why it's gone is the move to WebRender. Therefore it was removed for now in bug 1743310.
There is some discussion in bug 1612922 to add it back.