Yes, there is a 5s timeout in youtube’s code. However, it is not aimed at Firefox users.

The same code shows up on a fresh chrome installation without any extensions. And the code does not check for the user agent. So the 5s timeout is not there to make Firefox a bad choice for youtube. Following response to your link goes a bit deeper into what the code could mean: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/ka08uqj/