YouTube's playlist bar that shows up at the bottom of the page when you open a playlist or you click a video from your subscriptions has a new usage: displaying the list of liked videos after clicking "like" button. It's also used to display videos from a playlist after adding a new video to playlist.
If you've hundreds of liked videos, adding all of them to the bar takes a while and the browser is no longer responsive. Many YouTube users complaining about this new feature:
"Starting today, whenever I like a video, a bar pops up at the bottom of the page to tell me it was added to liked videos (a reminder that I don't need) - and then that bar freezes my browser and I can only close it with ctrl+alt+delete."
You can report this problem by clicking on "report a bug" at the bottom of the page.