Firefox stores HSTS entries in a SQLite database, which you can query by running:

  echo "SELECT * FROM moz_hosts WHERE type='sts/use';" | sqlite3 permissions.sqlite

from inside your profile directory.

To clear HSTS entries (which the "Clear recent history" UI does not delete), you can do:

  echo "DELETE FROM moz_hosts WHERE type='sts/use';" | sqlite3 permissions.sqlite

I've been periodically monitoring this database for HSTS supercookies over the last couple years and have yet to see any in the wild.