Are you saying Firefox shares data to Alphabet beyond Google as the default search engine? If so and if it applies to Sync (as if the question from OP here) can you please share sources for that?
You stated:
Mozilla does, sharing your account data
You also provided evidence that Mozilla uses Google Analytics trackers on the Firefox’s product information website. I mentioned that it’s not sufficient evidence of your claim as the trackers are independent of Firefox the browser and Sync. Additionally, the use of trackers for websites is clearly identified on Mozilla’s Privacy Policies and there is not much else mentioned on the Privacy Policies outside of those trackers and Google’s geolocation services in Firefox.
You’ve also mentioned Google’s contract with Mozilla, which is controversial for many people, but isn’t evidence of Mozilla providing user data to Google even in conjunction with the previously mentioned trackers. You then discussed various other browsers, but I’m not sure how that is relevant to your initial claim.
While it seems we can both agree that Mozilla and it’s products are far from perfect, it is looking like your initial claim was baseless as you have yet to provide any evidence of your initial claim. Do you have any evidence through things like code reviews or packet inspections of Firefox or Sync that hints Mozilla is sharing additional information to Google? At this point, I would even accept a user(s) providing evidence of some weird behavior like the recent issue where google.com wouldn’t load in Firefox on Android if someone could find a way to connect the weird behavior to Mozilla sharing data with Google.