It's a bit of a stretch to turn "may also" into "main purpose is", but you're right - that shows that indeed it's not a big leap to use it for advertising.
But no, as I understand it, this isn't extracting sensitive data from users and then only keeping it in anonymised aggregate form - the sensitive data is handled on your device and never reaches Mozilla, and the anonymised aggregate form (i.e. the high-level category derived from that data) is the only thing that's actually sent.
And again, it's always been an ad platform, it's still the only proven way to fund development.
I won't comment on this acquisition, cause I have no idea what this company does.