Basically I am using mull on android and librewolf on linux.I want to use firefox sync or something line that for syncing.So how good Firefox sync is, in privacy point on view. I am not anonymity paranoid I just want privacy so basically what do they collect and for what?
Mozilla pays for a premium subscription to Google Analytics, which allows them to opt out of data usage by Google. So, obviously Google still aggregates the data, but only for providing reports to Mozilla. Google may not use the data for their own user analysis/tracking, as they would do without the premium subscription. Otherwise, Google would be in breach of contract, which would be an easy lawsuit with high punishment for Google.
https://bugzilla.mozilla.org/show_bug.cgi?id=697436#c14