alt Hacker Newsor every time you click a YT link in firefox on android it asks "do you want to open this in the YT App?" where you're options are Yes (with an always use app checkbox) and "Cancel" to open in the browser. Like "Cancel" means "no, get out of the way and do what I want before you injected yourself in this flow"
You can disable the YouTube app's handler for YouTube URLs. In the app info, open by default. I haven't tried it, but presumably, all YouTube URLs will then open in your browser.