Firefox

Guys relax. Most of the 'research' comes from this reddit post: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/k9w3ei4/

It points out following code in youtube's polymer script:

setTimeout(function() {
    c();
    a.resolve(1)
 }, 5E3);

But exactly this code does show up on a stock installation of chrome too, and it does not check for the user agent. One of the responses goes a bit deeper into what the code above could do: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/ka08uqj/

It is rather clear, that this code is not aimed at firefox users to slow down their loading time.