It doesn't work for me either.
Operating System : Linux (Ubuntu 16.04)
Browser: Firefox Quantum (61.0.1 (64-bit))
lsof result:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
Enpass 9009 ipkiahme 30u IPv4 103963 0t0 TCP localhost:10391 (LISTEN)
Enpass 9009 ipkiahme 32u IPv4 103976 0t0 TCP localhost:10391->localhost:40854 (ESTABLISHED)
EnpassHel 15709 ipkiahme 20u IPv4 107067 0t0 TCP localhost:40854->localhost:10391 (ESTABLISHED)
It seems to work with Google Chrome, but Firefox one keeps saying "Connection Error"