【不明点あればコメントからご質問ください】
set.js
var firebaseConfig = {
apiKey: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
authDomain: "xxxxxxxxxxxxxxx.firebaseapp.com",
databaseURL: "https://xxxxxxxxxxx.firebaseio.com",
projectId: "xxxxxxxxxxxxxxxxx",
storageBucket: "xxxxxxxxxxxxxxxxxxx.com",
messagingSenderId: "xxxxxxxxxxxxxxxx",
appId: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
measurementId: "xxxxxxxxxxxxxxxx"
};
temp = JSON.stringify(firebaseConfig); //(1)JOSN形式に
localStorage.setItem('firebaseConfig',temp);
get.js
firebaseConfig1=localStorage.getItem('firebaseConfig')
firebaseConfig2=JSON.parse(firebaseConfig1) //(2)parse
console.log(firebaseConfig2);