The average user doesn't care about the underlying browser engine. Why should Mozilla add that to their advertising when no other browser maker on iOS is?

Err.. most other browsers on iOS do have an adblocker built in. It’s solely Mozilla dumping their users out in the cold.

These days ads are an attack vector. I’d never let my parents browse the web without a blocker.

The two most popular browsers on iOS do not support ad blocking, unless there's something I'm not aware of. I've never seen an ad blocker in iOS Safari or Chrome. Firefox on iOS has tracking protection which can block some ads if you believe the description of the setting.

Not real adblocking. If you care about your parents, get them a better device.

Not really. That's all Apple. Mozilla wants to try and provide the UI and experience and are doing so as best they can to satisfy their users requesting it.