How can I debug firewalld on CentOS 7 stopping running unexpectedly?
I don't have enough reputation yet to post a comment, but that was the idea.
Did you take a look at the firewalld logs ? I believe they are located in /var/log/firewalld.
If you don't find useful information in here, you could try to increase debug of the service by editing the /etc/sysconfig/firewalld file:
FIREWALLD_ARGS=--debug=10
This was suggested in this post.