Man, YouTube on Firefox has gotten really bad in the past few weeks. Every action on the screen takes a noticeable amount of time to complete, and it gets worse the longer I leave the browser open. Quitting and restarting makes it a little more bearable for a while, but it quickly degrades. Not sure if it's Firefox's issue or Google being dicks again, but either way it sucks.
I do use uBlock Origin, but have it turned off for YouTube (since I pay for premium). Wonder if Google is still making my life miserable simply because it's installed. (And no, I'm not going to switch browsers for this issue.)
