I haven't seen it yet either, not on firefox nor on chrome. From what I hear it's being rolled out gradually, so if you didn't get a notice yet, it doesn't mean that you won't get one in the future.

Youtube have been increasing their anti-adblock efforts dramatically in the last few weeks, changing the scripts multiple times a day. The ublock origin team is very quick in updating the filters though. They have a post on their subreddit with some details.