Ah, something I discovered when checking out "Sync" for the first time having updated to FF29. There was some info somewhere (can't remember exactly where I read it - sorry) that said the "old" Sync won't work with FF29 and later and the "new"sync doesn't work with pre FF29. You need to register at https://accounts.firefox.com/signup and login at https://accounts.firefox.com/signin or via >Options >Sync (the latter may be where I read it). HTH.