If telemetry was off by default, Mozilla would be blind to what the users are doing, to common sources of crashes and to what features are not being used.

Mozilla needs that information to be able to make a decision on where to focus.

Same for ads, default off ads defeats the entire purpose.

Mozilla is a privacy-aware and even -focused corporation and I will assume that the defaults of the browser are good for the common end user. Everything else may be configured.

mozilla needs to ask before collecting data. What is difficult about that?

They need to ask if they collect personal data as per GDPR or the US Data Privacy laws. If the data collected is not of private nature they don't need to ask.

Usage patterns can be granular enough to identify a user online, its easy enough to do that with machine learning nowadays. They have to ask anyway.

It doesn't send that granular enough of a usage pattern though, they have a group of people making sure that they data they collect cannot be used to identify someone.