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They had their ups and downs.
There was that thing where some domains where whitelisted from blocking, don't know whether it was cookies or something else. Not great, but easily explained by not wanting to break stuff for unexpecting users, maybe bad communication. Shouldn't happen when you go privacy first, but that was resolved quickly after being discovered at least.
There was the time when they injected affiliate links when visiting some sites, to generate some revenue of course. They overdid it and replaced affiliate links of other people I think, but again they changed it after the community complained. I don't know whether that's optional now or completely gone. In any case, no harm was done to the users in this instance.
One thing you can definitely hold against them to this day is their CEO. He supported anti-queer legislature in the past and was dismissed as Firefox CEO (CTO? Something very high up at least) for that reason. He did apologize for it and afaik didn't continue supporting that kinda stuff, but you never know.
Imo the browser as it is right now is pretty good and unique in what it has to offer. The biggest issue really is a lack of trust by the community.