Most of the posts/articles reference following reddit post: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/k9w3ei4/ . It shows the code from your screenshot. However the code does not check the user agent and is not injected server side (I checked by user agent spoofing and using a freshly installed chrome). So it will run on every browser and cannot be used against some specific ones.
This is not correct.
Most of the posts/articles reference following reddit post: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/k9w3ei4/ . It shows the code from your screenshot. However the code does not check the user agent and is not injected server side (I checked by user agent spoofing and using a freshly installed chrome). So it will run on every browser and cannot be used against some specific ones.
There is an answer to the post everyone seems to reference, which goes a bit deeper into what the code could do: https://old.reddit.com/r/firefox/comments/17ywbjj/whenever_i_open_a_youtube_video_in_a_new_tab_its/ka08uqj/