A new feature came with Firefox 44 that only recently started to come to my notice with Yahoo Mail offering to set up browser notifications for every time when a new email arrives there. This is something that I did not need and yet I did not get the option to switch it off permanently for that website so I was being nagged every time I when to check on things for that email address, an unneeded irritation. Other websites offered to set up similar push notifications but you could switch these off permanently so it is a site by site function unless you take another approach.
That is to open a new browser tab and enter about:config in the address bar before hitting the return key. If you have not done this before, a warning message will appear but you can dismiss this permanently. Once beyond that, you are presented with a searchable list of options and the ones that you need are dom.webnotifications.enabled and dom.webnotifications.serviceworker.enabled. By default, the corresponding entries in the Value column will be true. Double-clicking on each one will set it to false and you should not see any more offers of push notifications that allow you get alerts from web services like Yahoo Mail so your web browsing should suffer less of these intrusions.