yeah its not a 1click, but less intrusion, no buffering unless your device is prehistoric, if its a keeper, you can snip off kruft, and do all the neat VLC, and Handbrake things.
I don't think they are blocking based on browser. They are probably on another push. I'm getting "Ad blockers violate YouTube's Terms of Service" on lastest Firefox and uBO. I'm in Australia.
Using duckduckgo video search opens the video on the search results page, playing there plays without ads, I get the “sign in to confirm you’re not a bot” message sometimes though.
I doubt that I actively disabled webm. But yeah, it's been years of switching this or that, firefox adding or removing this or that, etc. That does add up. One of these pages seems to imply that "webm" was turned off when it first came out in firefox. And until not so long ago, Google didn't send everything in webm.