Normal way: Open URL or file, read/write bytes from it when available:
http://mxr.mozilla.org/seamonkey/source/netwerk/base/public/nsIChannel.idl
http://mxr.mozilla.org/seamonkey/source/netwerk/base/public/nsIStreamListener.idl
http://mxr.mozilla.org/seamonkey/source/xpcom/io/nsIInputStream.idl
http://mxr.mozilla.org/seamonkey/source/xpcom/io/nsIOutputStream.idl
http://mxr.mozilla.org/seamonkey/source/xpcom/io/nsIFile.idl
http://mxr.mozilla.org/seamonkey/source/xpcom/io/nsILocalFile.idl

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Listen to server sockets: (What I (ab)use)
http://mxr.mozilla.org/seamonkey/source/netwerk/base/public/nsASocketHandler.h
http://mxr.mozilla.org/seamonkey/source/netwerk/base/public/nsISocketTransportService.idl