First, you need to fill the address bar in the Firefox browser with this about:config to allow you to change the settings.
Your changes may be incorrect, here is the entire warning message:
Changing advanced configuration preferences can impact Firefox performance or security.
Warn me when I attempt to access these preferences

Find this setting and change to dom.webnotifications.enabled change value to false.
This setting will not display them.
You can delete all website notifications using the search Notifications on Firefox – Preferences and make change settings: about:preferences#searchResults.
Choose a notification option: Always ask, Allow or Block.
The message Block new requests for notifications sets requests when the webpage asks you to allow the notification.

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