This script is almost the same as https://github.com/qutebrowser/qutebrowser/blob/master/scripts/importer.py with the tweak that it add tags you added in Firefox bookmarks and prefixes them to the qutebrowser quickmark, which seemed to be missing in the original script.
Tested Firefox version: (64.0.2)
Export your bookmarks from Firefox as html.
For usage, see comments in file.