When importing bookmarks from Firefox ESR, if the source of the bookmarks is a places.sqlite file, the imported bookmarks are dumped into Unsorted Bookmarks->Firefox Import folder without the previous folder structure from the original browser. Importing from an HTML file generated by Firefox's Export feature works as intended, the original folders are preserved bellow the HTML Import folder in Falkon.
Patch made. Awaiting review. https://phabricator.kde.org/D28550