try using a script that looks something like this to launch it:
# get URL to load
url=$1; [ -z $url ] && url=about:blank
# try xremote first
$MOZILLA_FIVE_HOME/mozilla-xremote-client openURL\($url\) && exit 0
# if xremote failed, then launch the browser
exec $MOZILLA_FIVE_HOME/firefox $url
change "/opt/firefox" to where your mozilla/firefox is installed, and "firefox" in the last line to "mozilla" if its mozilla.