You can prevent Firefox from displaying prompts about accepting Notifications. There is a checkbox for that on the Settings page, use the Settings… button to the right of Notifications in the permissions section.
Enhanced Tracking Protection already blocks a number of the servers which deliver offers in pages, but if the offers are from a new server or the site itself, those typically are still allowed.
Fixing this is not trivial. A script in the page is modifying the visibility of various elements such as the dark translucent layer and whatever appears in front of it. But there is no obvious reason that pages shouldn’t be allowed to modify the visibility of elements. If you have used any ad blocking add-ons, you probably have noticed that they struggle to cleanly prevent this problem, sometimes leaving a dark layer with no close button (front-most content removed) or no scroll-bar (change to overflow properties not undone) or the page content invisible (change to opacity property or visibility property not undone).