Does the notification say that a website is sending the notification, or is it a popup from a Firefox extension? If it is from a website, try this: go to Settings -> type "Notifications" in the Settings search bar -> open Notification Permissions -> remove the website that you suspect is sending you notifications -> Save Changes and see if that stops the notification. If it is a popup, try temporarily disabling the Firefox extension and see if that stops the notification. If neither solves the issue, please provide a screenshot of the notification (if there is personal information, color over it).