As I have only just learned higher up on this page, FF has an Android button in the URL bar to switch from, for example, a Youtube page to the Youtube native application. I was missing this too!
I tried using FF on Android for a while. If I saw a page with a link to a YouTube video, the fastest way for me to watch the video was 1) Copy the link to the clipboard. 2) Open Chrome and paste it in 3) Click the link.
Lots of apps are bad actors like this - Facebook, Twitter, etc. In those cases, when you click a link in a post or tweet, you go there in their own built-in browser, trying to keep you in their app. But, they give you the ability to turn it off. It's in the app settings if you look.
Firefox doesn't have this. You have to wait until the page is fully loaded, and then decide if it's worth your trouble to open it in the native app now. Most of the time it isn't, it would have been faster in the native app, but you're there now, so eh. For YouTube, it usually is. But it's just so hair-pullingly frustrating.