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?
i dunno. it should. there’s another one a little deeper in the menus that does have a dismiss ‘X’ and it doesn’t come back after.