Update 1/16 - Adblock has informed BleepingComputer that its engineers fixed the problem and released ABP 3.22.1 and AB 5.17.1 on the Opera and Edge extension stores. The same versions are currently in review on the Mozilla and Chrome add-on stores, and should be made available soon.
I can attest it is very weird. The entire browser slows down on transition to full screen (lags like your machine has a virus or something) and introduces a buffer wheel of lag after the first 10 or so seconds of every video.