You haven't done much searching, have you? There are so many posts here on this topic.
This is the script I use, it even loads your home page for you if no url is specified...
Code:
#!/bin/bash
FIREFOX=[firefox executable path e.g. /opt/firefox-0.9.1/bin/firefox]
FF_REMOTE="-a firefox -remote"
url=$1; [ -z $url ] && url=`cat ~/.mozilla/firefox/*/prefs.js | awk '{if(index($0,"\"browser.startup.homepage\"")) {gsub("\"","",$2);gsub(");","",$2);print $2}}'`
$FIREFOX $FF_REMOTE 'ping()' && $FIREFOX $FF_REMOTE "openURL($url,new-window)" && exit 0
$FIREFOX $url