Yes, it turned out to be caused by ad blockers -- and there was actually a bug in AdBlock Plus that made things even worse.

Interesting how people were so quick to assume malicious intent against Firefox from Google.

Never used add-ons, but I do use a DNS blacklist.

Most of the ads I experience are pre-roll. In FF videos don't autoplay, so I just press F5 a few times until the garbage is gone. If an ad pops in the middle of something, I typically close the tab and find something else to listen to.