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?
Zen is a great fox based alternative if you want basically a reskinned Firefox.
It’s pretty good yes, but it also has that sign in to sync banner by default.
That’s weird, I’ve never seen it