Most of the articles writing about it seem to reference following reddit post: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/k9w3ei4/

Following code is pointed out:

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

While this is a 5s timeout, the code itself does not check for the user agent. So wherever the code is the 5s timeout will occur. The code also does not seem to be injected server side. I spoofed my user agent and for good measure installed a fresh google chrome, both times the code was present. So this code cannot be used to make any browser slower without making the other browsers slow too.

There is a response to the reddit post, which most articles seem to take their intel from. IMO this response does a good job at exploring what the code could be used for and points out that it is more than likely not for slowing down Firefox users: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/ka08uqj/

I am amused by thinking that many journalists seem to take this story from a post on reddit, without even reading the direct responses - or just copy from another article.