Not really. They have a strong financial interest to have as many eyeballs as possible watching YouTube, because only then can they show ads.

I don't see how breaking YouTube for a portion of the world population is a sensible part of that plan.

For most people, if YouTube does not work in Firefox, they will not switch to a different video platform. They will switch to Chrome.

A lot of people have Chrome installed alongside Firefox or Edge or whatever and will just try the video somewhere else. So YouTube actually gets more revenue and Chrome gets free advertising. If a major website works on Chrome but doesn’t work correctly on Firefox most users will tend to blame the browser instead of the website and think “I stopped using Chrome because of the memory issues but at least it’s reliable.” The logic even sounds superficially reasonable.

It's a trade-off. Which way do they gain more money? From showing the YouTube ads to Firefox users, or from getting users into the Chrome ecosystem for however that is monetized?

Making the web buggy on firefox means we're in the extinguish phase for alternate web engines. Soon there will only be chrome based browsers. And then once that happens, they'll make it harder and harder to build alternative browsers around chrome.

Microsoft will fork Chromium in that case.

Maybe, but it wouldn't help much: They too would be happy to get rid of ad blockers.