A user-agent should give users agency, it doesn't have to make all decisions for them. It wouldn't necessarily be bad of firefox to block more ads, but I'm perfectly happy with it blocking only the clearly malicious content like pop-ups, trackers, fingerprinters, crypto-miners and leaving the rest for users to decide themselves.

Adblocking isn't a solved problem. Current state of the art borks a minority of websites. The user must diagnose that the problem is caused by adblock and either abandon their intended browsing or manually disable adblock.

Plenty of ordinary users don't mind ads. They'd be frustrated to be told there's now a "don't break the website" switch.

Adblocking requires very frequent updates. It's a cat-and-mouse game. Even if Firefox just installed UBO by default plenty of users would now start filing bugs on Firefox's tracker.

Part of the difference is that every browser blocks popups, so sites are incentiviced to fix the resulting breakage. But on sites run by people who don't care about usability (like some governments and banks) you'll still see messages informing you to allow popups.