Since Firefox version 56 (not the ESR version), Mozzila changes the default dbm format from files cert8.db, key3.db and secmod.db to sql format cert9.db, key4.db and pk12mod.db. These new format were not recognized by LO, only the dbm format or from some Certificate Manager, like as Kleopatra that is called when “Start Certificate Manager” button is pressed.
The is another “command-line” (hard) alternative by using the libnss3-tool package. This package installs certutil and pks12util. certutil and pks12util handle the both format (dbm and sql). You can export the certificates from firefox and create the dbm files (cert8.db, key3.db, etc) at same path of your firefox profile and all certificates will be available to LO, after pressing “Sign Document”.