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If you are using the open source Firefox web browser to browse YouTube and watch its videos, then you might have noticed that there is an artificial delay
Google slows down Firefox users when watching YouTube…
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/
That doesn’t explain why it stops happening when you change your user agent in Firefox.