I assume that you want the request headers, not the response headers. Then you register an observer for notification http-on-modify-request. General documentation https://developer.mozilla.org/en/Observer_Notifications#HTTP_requests , sample code can be found at https://developer.mozilla.org/en/Creating_Sandboxed_HTTP_Connections#Handling_cookies . Getting headers is easy, you just call nsIHttpChannel.getRequestHeader().
, http-on-examine-response , , http-on-examine-cached-response.
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