Thanks for posting, @bhelden - definitely want to make the video watching experience more natural in Firefox. Would you mind telling me which version of Firefox you have and whether you're using it on desktop or mobile?
As far as the latest version on desktop (112.0.1), the cursor does disappear after a few seconds once full-screen is triggered on YouTube. I can help you file a bug ticket on bugzilla.mozilla.org/ to flag this to our media team.
I am running the latest version of Firefox (112, 64 bits) on macOS Big Sur. The cursor doesn't disappear at all while watching a Youtube video. I just tried it again. It's good to hear that it should behave the opposite way thought.