code snippet to fetch data from sheets

 function getAllData() {

  var ss = SpreadsheetApp.getActiveSpreadsheet();

  var sheets = ss.getSheets();

  var sheet = ss.getActiveSheet();

  var firebaseUrl = "insert_your_url";

  var base = FirebaseApp.getDatabaseByUrl(firebaseUrl);

  var dataSet = [base.getData()];

  

  // the following lines will depend on the structure of your data

  var rows = [],

      data;

  for (i = 0; i < dataSet.length; i++) {

    data = dataSet[i];

    for (j = 0; j < dataSet[i].length; j++) {

      //check your data tye in firebase and update accordingly 

      //for eg : my data types are id,name,batch 

    rows.push([data[j].id,data[j].name,data[j].batch]);

    }

  }

  dataRange = sheet.getRange(2,1, rows.length,3 );

  dataRange.setValues(rows); 

}