Create a script that looks something like this and run it to launch firefox:
url=$1; [ -z $url ] && url=about:blank
$MOZILLA_FIVE_HOME/mozilla-xremote-client openURL($url) && exit 0
exec $MOZILLA_FIVE_HOME/firefox $url
Change /opt/firefox to whereever your firefox is installed. Mozilla in Redhat/Fedora uses a similar script as well.