Is it possible to send a synchronous request in the Firebase?

There is no way to synchronously load data from the Firebase Database.

While it is common for developers new to Firebase to wish for a synchronous method, it simply doesn't fit with Firebase's data synchronization model. Also see my answer here: Setting Singleton property value in Firebase Listener

It is not possible to load data synchronously with the official SDK. However, you can access all the data in firebase using the REST API. This would allow you to make synchronous calls. As mentioned about, Firebase is a realtime database and you will be missing the feature of updates when your data changes.