Only way to disable this is to open about:config and change some value. Why this has no dismiss button?
There is couple more settings that are on by default and i hate those; but mostly those are subjective. However who wants 80px long "SIGN IN SO WE STORE YOUR BOOKMARKS, HISTORY IN OUR SERVERS" stuff on their screen?
Firefox sync is end to end encrypted by the way: https://support.mozilla.org/en-US/kb/sync
It is technically also self hostable, but I gave up when I was investigating. Hosting the sync server itself wasn't too bad, but I couldn't figure out how to also self host the account server, which is also needed to have the sync process entirely based on my own infrastructure.