> But big daddy Google would get so cross

I don't think Google would be as mad as every other publisher on the internet who relies on ads for their living. They would go on campaigns to block Firefox from their websites entirely. They already rail against adblockers with full-site popups and blocking content until you disable your adblocker, making adblocking a default would escalate the situation exceptionally quickly.

Firefox's marketshare would drop even more quickly if people couldn't visit their favorite sites with it (and were likely directed to Chrome or Edge).

Without an alternative, readily available method of monetizing websites, it's realistically going to have to remain opt-in (i.e. installing uBlock).