IIRC, the complaints I've read online are along the lines of: Mozilla ran a Mr Robot ad campaign with browser integration, partnered with an ad company for some research in a seemingly-sneaky way, made their CEO resign (now the CEO of Brave) because he donated personal money to an anti-gay marriage lobby effort, forced pocket integration for ad revenue purposes, and they deprecated their extension api which broke most extensions that did not get updated.

I'm not saying anything in favour of or against these decisions, they're just what I remember reading people complain about over the last year.

edit: typo and clarification