Kill

Fixed in the latest update 1.21.74.

Same here Ubuntu 17.10 Firefox 60.0.1

Did manage to attach gdb to the process and get a backtrace if it helps:
 

gdb -p 2556

Attaching to process 2556
[New LWP 6890]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fd135203951 in __GI___poll (fds=0x7ad240, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
29      ../sysdeps/unix/sysv/linux/poll.c: No such file or directory.
(gdb) bt
#0  0x00007fd135203951 in __GI___poll (fds=0x7ad240, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007fd13207c169 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fd13207c27c in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fd135f53f07 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /opt/Enpass/libs/libQt5Core.so.5
#4  0x00007fd135f0276a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /opt/Enpass/libs/libQt5Core.so.5
#5  0x00007fd135f0a5fd in QCoreApplication::exec() () from /opt/Enpass/libs/libQt5Core.so.5
#6  0x0000000000403b1c in main ()