Postman pre-request script to create a Firebase authentication JWT header.

firebase_pre-request_script.js

/**
 * This script expects the global variables 'refresh_token' and 'firebase_api_key' to be set. 'firebase_api_key' can be found
 * in the Firebase console under project settings then 'Web API Key'.
 * 'refresh_token' as to be gathered from watching the network requests to https://securetoken.googleapis.com/v1/token from 
 * your Firebase app, look for the formdata values
 * 
 * If all the data is found it makes a request to get a new token and sets a 'auth_jwt' environment variable and updates the 
 * global 'refresh_token'.
 * 
 * Requests that need authentication should have a header with a key of 'Authentication' and value of '{{auth_jwt}}'
 *
 * Currently the nested assertions silently fail, I don't know why.
 */
pm.expect(pm.globals.has('refresh_token')).to.be.true;
pm.expect(pm.globals.has('firebase_api_key')).to.be.true;

var sdk = require('postman-collection'),
  tokenRequest = new sdk.Request({
    url: 'https://securetoken.googleapis.com/v1/token',
    method: 'POST',
    body: {
      mode: 'urlencoded',
      urlencoded: [{
          type: 'text',
          key: 'key',
          value: pm.globals.get('firebase_api_key')
        },
        {
          type: 'text',
          key: 'grant_type',
          value: 'refresh_token'
        },
        {
          type: 'text',
          key: 'refresh_token',
          value: pm.globals.get('refresh_token')
        },
      ]
    }
  });

pm.sendRequest(tokenRequest, function(err, response) {

  pm.test('request for access token was ok', function() {
    pm.expect(response).to.be.ok();
  });

  const json = response.json();
  pm.expect(json).to.an('object');

  pm.test('response json has needed properties', function() {

    pm.expect(json).to.have.own.property('access_token');
    pm.expect(json).to.have.own.property('token_type');
    pm.expect(json).to.have.own.property('refresh_token');

    const accessToken = json.access_token;
    const tokenType = json.token_type;
    const refreshToken = json.refresh_token;

    pm.environment.set('auth_jwt', tokenType + ' ' + accessToken);
    pm.globals.set('refresh_token', refreshToken);

  });

});

How we can import this in postman app ?

thank you so much @moneal!
@rahulvyas copy and paste the script into Pre-Request Script of either your request or your whole collection. Thus the script will be run everytime you trigger the request and will provide you a valid jwt. Make sure to fix the test as suggested by @roboworski

You can also remove this piece tokenType + ' ' + of this line pm.environment.set('auth_jwt', tokenType + ' ' + accessToken);. You can then use {{auth_jwt}} as token of authorization method "bearer". This is especially helpful to inherit the authorization method to all requests within a collection.