Define global firebase constant in Typescript

Question

I have a service worker written in Typescript that does this:

export default null;
declare const self: ServiceWorkerGlobalScope;
declare const firebase: any // ### FIX THIS. This needs to be typed as firebase ####

// the imports below expose firebase global object
self.importScripts('https://www.gstatic.com/firebasejs/8.9.1/firebase-app.js');
self.importScripts(
  'https://www.gstatic.com/firebasejs/8.9.1/firebase-messaging.js'
);

self.addEventListener('install', function () {
  // do stuff here.
  firebase.initializeApp();
});

It's defining firebase as a const of type any. I want it to be the same type one gets by doing

import firebase from 'firebase/app';

I would like to not import/bundle firebase in my service worker, because that significantly bloats my service worker. The import scripts already expose everything I need (i.e., they expose a global firebase for me).

I was doing this before, but webpack makes the file from 800 bytes to 80kb.

import firebase from 'firebase/app';

How can I get the types of firebase without importing all the code that comes with firebase/app?

I've tried this:

export default null;
declare const self: ServiceWorkerGlobalScope;
/// <reference types="firebase/app" />
declare const firebase: firebase;

But I get the error Cannot use namaespace 'firebase' as a type.