I know. But I want to open it in the YouTube app in part because I live in a rural area with DSL. The YouTube app opens much faster. But that icon in the URL bar doesn't appear until the page is completely loaded. So it doesn't speed up the experience at all for me. I get to wait for the entire site to load in Firefox, then I get to wait for YouTube again once I click the button.

Wow, thanks for the tip. I've been using Firefox on Android for years and never noticed that. I've immediately tried it out and it works fantastic. One more reason to keep on using Firefox!