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?
You may prefer to skip Mozilla entirely and self host your own Firefox Sync: https://github.com/mozilla-services/syncstorage-rs
Otherwise, i've been using Firefox Sync for years and haven't had a single issue with it.
Regarding data sharing, do note that your Firefox Account email address is shared with Troy Hunt (haveibeenpwned) via the Firefox Monitor service, so Firefox can warn you if you have suffered a data breach. Deleting your account is the only way to opt out of that. https://support.mozilla.org/en-US/kb/how-do-i-opt-out-firefox-monitor
I personally have no issue with it as he's a well known security consultant - BUT the caveat being that he's a Microsoft regional director, you couldn't know if that data is being shared any further.
Bruh, I didn't know about this, I swear we can self host almost anything, can we self host the Internet?
As far as i know it's barely possible to really host it yourself. I don't know the details anymore but i looked into it half a year ago and quickly gave up. Think it was severely outdated and depedency issues, not sure anymore.
I set it up using a docker image based on the older Firefox sync repo. It's outdated but it works. What I don't self host is authentication as it is way more involved than I prefer my self hosting projects to be and I'd probably end up frustrated by some little thing not working.