How to fix AngularFire migration observable error type?

Question

I upgraded a project that I haven't touched in more than a year. It was at angular 10. I upgraded to angular 12 but now I need to upgrade AngularFire from v6 to v7 and it seems i have a typing problem (at first sight).

here are my dependencies :

"@angular/animations": "~12.2.16",
"@angular/cdk": "^12.2.13",
"@angular/common": "~12.2.16",
"@angular/compiler": "~12.2.16",
"@angular/core": "~12.2.16",
"@angular/fire": "^7.2.1",
"@angular/flex-layout": "^12.0.0-beta.35",
"@angular/forms": "~12.2.16",
"@angular/material": "^12.2.13",
"@angular/platform-browser": "~12.2.16",
"@angular/platform-browser-dynamic": "~12.2.16",
"@angular/router": "~12.2.16",
"date-fns": "^2.22.1",
"eslint": "^8.10.0",
"firebase": "^9.4.0",
"ngx-material-timepicker": "^5.5.3",
"rxjs": "^6.6.6",
"tslib": "^2.1.0",
"tslint": "^6.1.3",
"zone.js": "~0.11.4",
"rxfire": "^6.0.0"

I have a pretty simple non working example :

import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/compat/firestore';
import { Order } from '@models/order';
import {  Observable } from 'rxjs';

@Injectable({
  providedIn: 'root',
})
export class OrderService {
  private ordersCollection: AngularFirestoreCollection<Order>;
  orders: Observable<Order[]>;

  constructor(private firestore: AngularFirestore) {
    this.ordersCollection = firestore.collection<Order>('orders');
    this.orders = this.ordersCollection.valueChanges();
  }

 ...Some CRUD methods bu im not even there for the migration...
}

On the variable this.orders I have the following error :

Type 'Observable<Order[]>' is missing the following properties from type 'Observable<Order[]>': _isScalar, _trySubscribe

I strictly followed the migration guide of AngularFire and read the new documentation implementation without any success...

Here is the migration page : https://github.com/angular/angularfire/blob/master/docs/version-7-upgrade.md

Does anyone have any tips on that?

Thank you very much in advance and sorry for the possible typos.

Do not hesitate if you need more informations.

Regards.

Alex