On Reddit there are two separate threads about this: https://www.reddit.com/r/firefox/comments/3xhxuf/the_youtubecom_website_not_the_video_player/ https://www.reddit.com/r/youtube/comments/3lcxvj/firefox_started_freezing_on_youtube_a_lot_in/ It sounds like we slow down significantly when the user is subscribed to a high number of channels. I don't think anybody has figured out where the threshold is when things start to get bad, but it sounds like everybody who has seen this has been subscribed to somewhere between 500 - 1000 channels. I'm filing this under Layout under the assumption that it's got something to do with rendering the markup that YouTube is sending, but it would be nice to have somebody create solid STR and be able to give us better data so that we can pinpoint this issue. Requesting QA in the hopes that we can get this narrowed down and properly triaged.

Looks like this was previously filed, marking as dup.