You could use FF Sync with a simple setting
https://librewolf.net/docs/settings/#enable-firefox-sync
Or you can do it by just exporting to a file and importing
1. Save bookmarks as JSON file in FF
Bookmarks -> Manage Bookmarks -> Import and Backup ->Backup
2. Open LW and restore file you created above
LW Bookmarks -> Manage Bookmarks -> Import and Backup ->Backup->Choose File