Why would it not be private? Yes data is sent to the servers but the trust that you must have that they don't keep the data is not different from the trust you must have in your password manager or encrypted email or backup storage provider.

All previous similar issues got hand waved with "Mozilla has to make money somehow". I am afraid one day remaining users will decide that Chromium based browsers provide a better experience in most use cases, and now have same privacy guarantees as Firefox. Looking at market share maybe that had already happened and pragmatism won over ideology, and we will all suffer the consequences of such monoculture.