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?
Sync is E2EE, but also you can sync bookmarks without history or whatever works. I’m always a little confused how people manage using multiple devices without having bookmarks synced across?