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?
As others have said, it’s quite good on privacy. For the truly paranoid, IIRC you can even self-host the sync server.
From the security perspective of privacy, do make sure to use a good password for the Mozilla account, the account password is also the encryption key for the E2E encryption.
They could have separated the two passwords or at least provide an option to do so
If the user skips the encryption key set up page, then they can make the primary password to be the encryption key