OK, now I am having the same issue on my Linux box (Ubuntu 17.10).
/opt/Enpass/bin/EnpassHelper/EnpassNMHost /home/xxxx/.mozilla/native-messaging-hosts/EnpassNMHost.json firstname.lastname@example.org
The process is consuming 100% of a CPU. When I close firefox, the process continues to consume CPU. When I start firefox again, a new (normally behaving) process starts, and the old one continues to consume CPU until it is killed.
I have tried to strace a misbehaving process, but it's not doing anything but context switching (visible in top).
Just let me know what to run to get you a diagnostic dump, I'm all for it.
EDIT: looks like closing firefox while EnpassNMHost is misbehaving results in a crash of firefox (observed by firefox wanting to submit a crash report).