Getting ReferenceError with providers

I’m trying to use Firebase with Ionic. My instructor has provided a file that we can use as a provider, which is:

import { Injectable } from '@angular/core';
import 'rxjs/add/operator/map';

declare var firebase: any;

@Injectable()
export class FirebaseProvider {

	constructor() {
		let config = {
			apiKey: "",
			authDomain: "",
			databaseURL: "",
			storageBucket: "",
			messagingSenderId: "" 
		};
		firebase.initializeApp(config);
	}

	db() {
		return firebase;
	}
}

The fields from config are populated with our Firebase credentials.

I imported both in app.ts and login.ts (where I’m trying to use the API), but I get this on console when I serve the app:

EXCEPTION: Error: Uncaught (in promise): EXCEPTION: Error in ./LoginPage class LoginPage_Host - inline template:0:0
ORIGINAL EXCEPTION: ReferenceError: firebase is not defined
ORIGINAL STACKTRACE:
ReferenceError: firebase is not defined
at new FirebaseProvider (http://localhost:8100/build/js/app.bundle.js:197:9)
at http://localhost:8100/build/js/app.bundle.js:35959:45
at ReflectiveInjector_.instantiate (http://localhost:8100/build/js/app.bundle.js:29494:27)
at ReflectiveInjector
.instantiateProvider (http://localhost:8100/build/js/app.bundle.js:29435:25)
at ReflectiveInjector
.new (http://localhost:8100/build/js/app.bundle.js:29424:21)
at ReflectiveInjectorDynamicStrategy.getObjByKeyId (http://localhost:8100/build/js/app.bundle.js:29076:50)
at ReflectiveInjector
.getByKeyDefault (http://localhost:8100/build/js/app.bundle.js:29604:38)
at ReflectiveInjector
.getByKey (http://localhost:8100/build/js/app.bundle.js:29576:25)
at ReflectiveInjector
.get (http://localhost:8100/build/js/app.bundle.js:29385:21)
at ElementInjector.get (http://localhost:8100/build/js/app.bundle.js:31156:48)
ERROR CONTEXT:
[object Object]

A not so quick Google search showed people with similar issues with a Google Maps thing, but I had no luck.

My system information:

Cordova CLI: 6.3.1
Ionic CLI Version: 2.0.0
Ionic App Lib Version: 2.0.0
OS: Distributor ID:    Ubuntu Description:    Ubuntu 16.04.1 LTS 
Node Version: v4.2.6