Firefox 2.0.0.14/Gecko
You are maybe wondering why your Facebook chat is not working anymore with Firefox? You probably got the last version of Firefox which is 2.0.0.14. This last version changed their userAgent tag. This tag was used to identify the Browser. Before you could read "firefox" in it. Now it is written "Gecko".
You have to wait for the websites to change their code in order to let your new Gecko browser access the new stuff...

This is my userAgent code now:
"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.14) Gecko/20080404 NET_mmhpset"