In the stock browser, going to a youtube.com url offers a choice of opening the url in the YouTube application. This doesn't happen in Nightly, it just opens the youtube page.
This is different from bug 700942. That bug is about embedded youtube video, which doesn't behave that way in any browser on android. This report is about links that point to youtube.com, which behaves in the way described in this report in the stock browser.
We already discussed this a year ago (no idea what bug) and decided not to do this