import { AngularFirestoreModule, FirestoreSettingsToken } from '@angular/fire/firestore';
@NgModule({
  declarations: [AppComponent],
  imports: [
     ...
     AngularFirestoreModule,
     ...
  ],
  providers: [{ provide: FirestoreSettingsToken, useValue: {} }],
  bootstrap: [AppComponent]
})
export class AppModule { }