The thing is, I really don't think, Google would care about Firefox. Firefox is sitting at negligible percentages of usage share. The only real competitor to Chrome is Safari and that's because of iOS.
I guess, they might impact Safari on macOS with this, but someone would have to try this out to actually see, and ultimately, this could still just be a dumb mistake.

Having said that, Google holds a near-monopoly in both video content and web browsers. They have a special duty to not disadvantage competitors and even if this was an honest mistake, I do think, it deserves a slap on the wrist.