Select Security and Privacy, then select Site Settings.
Go to the Permissions tab and select Notifications.
Select the option Don't allow sites to send notifications.
Click the back arrow next the the Notifications header.
Go to the tab Content and select Pop-ups and redirects.
Open Firefox settings, which can be accessed in the drop down by clicking the the dashed lines in the upper right hand corner of the browser window.
Select Privacy & Security.
Scroll down to the Permissions tab and select the Settings... button for Notifications.
Click on Remove All Websites.