It should be /etc/sysconfig/openfire - add / extend there:
export OPENFIRE_OPTS="-Djava.awt.headless=true -Xfoo"
Then starting Openfire should fail as the "-Xfoo" parameter is unknown - if this is the case remove "-Xfoo" and start Openfire.
I assume that you did configure here the JAVA_HOME for Oracles JDK - otherwise a more or less random JRE is used by Openfire.