nsIFile

tested on Firefox 7.0.1

const Ci = Components.interfaces;  
const Cc = Components.classes;

function ReadWrite( data ){
    
        	try{
                            Components.utils.import("resource://gre/modules/NetUtil.jsm");
                            Components.utils.import("resource://gre/modules/FileUtils.jsm");

                            var file = FileUtils.getFile("ProfD", ["FileName.txt"]);
                            var ostream = FileUtils.openSafeFileOutputStream(file );
                            var converter = Cc["@mozilla.org/intl/scriptableunicodeconverter"].
                                                     createInstance(Ci.nsIScriptableUnicodeConverter);

                            if( data ){
                            
                                    if ( !file.exists() ){
                                            file.create( Ci.nsIFile.NORMAL_FILE_TYPE, 420);
                                    }
                                    converter.charset = "UTF-8";
                                    var istream = converter.convertToInputStream(data);
                                    NetUtil.asyncCopy(istream , ostream, function(status) {
                                                    if (!Components.isSuccessCode(status)) {
                                                            alert( "Error! :" + status );
                                                            return false;
                                                     }
                                    });
                                                                        
                           }else{
                                    var data = '';
                                    var fstream = Cc["@mozilla.org/network/file-input-stream;1"].
                                                  createInstance(Ci.nsIFileInputStream);
                                    var cstream = Cc["@mozilla.org/intl/converter-input-stream;1"].
                                                  createInstance(Ci.nsIConverterInputStream);
                                    fstream.init(file, -1, 0, 0);
                                    cstream.init(fstream, "UTF-8", 0, 0); // you can use another encoding here if you wish

                                    let (str = {}) {
                                          let read = 0;
                                          do { 
                                                read = cstream.readString(0xffffffff, str); // read as much as we can and put it in str.value
                                                data += str.value;
                                          } while (read != 0);
                                    }
                                    cstream.close(); // this closes fstream
                                    return data;

                               }

	}catch(e){
//                          alert( e);
                            return false;
	}
    }