Q:

I can't see the iptables rules where firewalld enables helpers.  Why?  Have the helpers been disabled somehow? 

Here is an example of the ftp helper added by enabling the ftp service in the public zone:
# iptables -t raw -S | grep CT
-A PRE_public_allow -p tcp -m tcp --dport 21 -j CT --helper ftp

https://firewalld.org/2016/10/automatic-helper-assignment

I have not tried to disable any of the default helpers, but running the above command does not show any such rules.   Nor does sudo iptables-save | grep 21.   Why? 

# rpm -q firewalld
firewalld-0.5.5-1.fc28.noarch

A:

Here is an [example] of the ftp helper added by enabling the ftp service in the public zone. 

It sounds like the ftp "helper" is only enabled, if you specifically allow the ftp "service" in firewalld.   For each helper definition, there should be a matching service definition. 
Originally I thought you would only enable a "service" if you were running e.g. an FTP server.   FTP passive mode, which is preferred nowadays, still requires an extra connection.   The client must connect to the server on a second port, to transfer file data.   The "helper" is needed to detect which port is being used and allow this extra connection. 
None of the helpers are marked as -client (like the service dhcpv6-client).   But as far as I can tell, enabling the FTP service on a client will allow it to use the original FTP active mode (assuming firewalld is the only firewall in the way).   I think this example is not a big security problem, but it could be very confusing :-(. 
(Nitpick: FedoraWorkstation will happen to allow FTP active mode anyway, because it allows incoming connections to any port above 1024.   Please do not ask me to explain why). 
The second example I looked at was the irc helper, but I cannot understand it.   It looks broken to me. 
# cat /usr/lib/firewalld/helpers/irc.xml
<? xml version="1.0" encoding="utf-8"? >
<helper module="nf_conntrack_irc" family="ipv4">
  <port protocol="tcp" port="194"/>
</helper>
# cat /usr/lib/firewalld/services/irc.xml 
<? xml version="1.0" encoding="utf-8"? >
<service>
  <short>IRC</short>
  <description>An IRCd, short for Internet Relay Chat daemon, is server software that implements the IRC protocol.</description>
  <port protocol="tcp" port="6667"/>
</service>

