If you use the Namoroka version of Firefox then chances are that Facebook might not be showing you the option to chat (i.e go Online/Offline). Mind you, it has nothing to do with Facebook blocking you from using chat, it is because Facebook doesn’t recognize Namoroka as a browser good enough to support Facebook chat.
I say that Facebook chat is not good enough to be used in a browser like Namoroka!
When you log-in to Facebook through Namoroka who will not see the option to go online/offline, here is a quick fix for that:
Step 2: Search for general.useragent.extra.firefox. Once found double click on it.
Step 3: In the dialog box which shall appear replace whatever is written with Firefox/4.
Step 4: There is no step 4! That was all. Now go to Facebook and see for yourself if the chat box is there or not. It will be there unless of course Facebook has banned you from using Facebook chat.