Allow User access right, so that user can create a web push notification.

In General Setting, just past firebase keys and config code (First Config Firebase Setup)

Web Push Notification Menu and Tree View.

Web Push Notification Form view, Here you can create a notification.

After 'Validate' notification click 'Send Notification', and users get the notification.

Make sure your users allow the Notification popup in their browsers.

In 'Register User' here you can see the list of all users who are getting notifications.

Web Notification look like below image. Cheers....

Goto Firebase Console and click on add project.

Here add your project name and click next.

If you want google analytics so enable it else disable that and click continue.

Click continue.

After adding project, Select 'Web' option click on it.

After that step here just add your project nick name and click register add.

Then next all step and click 'Continue to Console'.

Now here goto project setting.

Here goto 'General' tab and copy 'config' code (make sure only copy selected code as per indicate in this screenshot).

Now goto 'Cloud Messaging' and copu server key and past it in 'Api key' in odoo general setting. Now click on 'Generate Key Pair'.

After that copy that key and past in 'Vapid' in odoo general setting. Thats it.