

Firefox automatically updates to install new privacy-abusing features like this one: https://news.ycombinator.com/item?id=40974112. So getting all the settings right once isn’t enough—you need to be constantly vigilant.
Like you said, a fork is a much better option, since they take care of stripping out the user-hostile nonsense for you. I like Librewolf.
K-9 mail is literally Thunderbird. It’s been rebranded and taken over by Mozilla. They’re keeping the k-9 branding as its own (otherwise identical) app as a nostalgia token for the people that have used it for a long time.