The gBrowser.addTab function is what you want. One of the parameters you pass to this function is postData , and allows you to set postData as you would like. The MDN documentation for this function also points to an article on preprocessing POST data . If I read this second article correctly, the POST data should be passed as nsIInputStream (specially created as nsIMIMEInputStream ). The article provides an example of a code snippet for converting from a standard GET-style format string (example: foo=1&goo=somestring ) to the intended format.
Edit: So, to use your example, you can do something like this:
var myData = "a=NOMADE&b=NOWAY&another=IDONTKNOW";
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