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?
(how) can i sync my stuff with LibreWolf?
You can still use Firefox sync if you want to sync
As someone else said firefox sync does work, but if you are using LibreWolf for privacy reasons that may not appeal to you. So, just as an alternative you can just open the passwords folder, and export a csv file with all of your passwords. Encrypt it and put it on a microsd card and stick it somewhere. Now you’ve got a backup of your passwords without needing to give them to firefox.