Firefox stores bookmarks for each profile in a SQLite file,
in the .mozilla/firefox/<profile>/ directory, in the users home
directory.

If the user has no profiles, don't display the 'Firefox' option in
the impport combo box. If he has one profile, automatically import
from that one. If he has multiple profiles, display a dialog which
allows the user to select the profile he wants to import from.

https://bugzilla.gnome.org/show_bug.cgi?id=772423