zzz
I could be fine with Firefox showing ads based on generic tracking that is non-invasive.
The problem is that if there's any server-side tracking that could be modified, I don't get to confirm that server-side code and there could be a demand made by the government to modify the server code.
So as an example, if the browser monitors what websites I go to, assigns a category to them on the server such as "technology" and "golfing," and then shows ads, I don't mind that. But if the government then demands that all ping times are recorded and Mozilla retain all site history in secret, that is not something I want. Also, if the government requires Mozilla to make these changes, they woukdn't be able to tell is.
It's very hard to trust a privacy-based US company because any time you rely on the servers as being private, you are placing trust in them not being required to be compromised.
Because Mull is just a fork on Firefox with telemetry removed, you don't have to have concerns with Mull.
Firefox is a good browser so if your threat model does not include the government (no illegal activity, not a minority group that could be persecuted such as LGBT or minority religious group or protester with minority that could become persecuted, no expected illegal activities in the future, no.one within the government upset with you and no anticipation of that happening and mostly moderate views or slightly right or left of center) then please use Firefox since they need the money since that judge's disasterous anticompetitive decision that Google is a monopoly because it subsidizes Mozilla by paying for it to be the default search, resulting in the possible destruction of Mozilla and the only non-Chromium based browser that holds back Google controlling all web standards and making them all pro-tracking.
Mull also has built in default hardening. It's a good browser.