So you're expecting Google engineers and managers to prioritize adding broad cross-browser support, which adds more work for them, when the same company is also developing a competing browser?

No, Firefox always been a second-rate guest at Google properties, and I'm not expecting it to change soon either. Why would they make it better when status quo means more Chrome users (in their mind)?

It's not 2003, the browser isn't the battle.

I would expect YouTube managers to pressure the Chrome managers, because YouTube brings in billions of dollars every month. Likewise I would expect the trend to move in favor of YouTube, because the browser loses money at an increasing amount and YouTube generates money at an increasing amount. 70% of YouTube happens on Mobile, and in the US more people are now watching on TVs than phones. Source: Nielsen, the old-school company that has huge influence over ads.

The site pops a literal warning saying "having problems? turn off your ad blocker" so I'm not sure where the mysteries lie here.

They're testing on thousands of devices. And they're probably even testing against ad-blockers on your bro-browser. But they're certainly not motivated to optimize that experience, so you get what you get.