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?
Your syc data is secure, yes, but not so your account data, because is also known by Google, amon yor IP other. Enough to track you through every page with Google APIs (most) also your email, whichi is an unique identificator in the web. Don’t trust any web or service which share data to third parties, less in US webs or services. This is one of the reason because I always prefer EU soft, webs and services. They are also not perfect respect privacy, but lightyears better as those from US companies, where something like privacy laws, GDPR and user protection don’t exist or only in very deficient manner.
Technical point: US companies still need to abide by the GDPR when the user is in the EU or UK.
(But yes, I accept your main point - that protection is not shared to US citizens of those same companies who operate two very different levels of distinction. European originating software/services usually operate at the higher level of protection across all users. )