He is talking about when Mozilla made Firefox send the browsing history of randomly selected German users to a company they had invested in, Cliqz. That is the by far biggest ethical violation of Mozilla and which, justifiably, still haunts them. That said both Chrome and Edge are worse.
Browsing history is something which is very easy to deanionymize so I do not buy that defence. What they did was inexcusable, but again both Microsoft and Google are more evil. I run Firefox, but only as the lesser evil.
Edit: 1% is 1% too many if you have a company which claims to care about the privacy of their users. Especially since browsing history is very sensitive and easy to deanonymize.
