Firefox settings -> type in "permissions" -> open Settings for Notifications -> block specific websites or select "Block new requests asking to allow notifications" -> Save Changes

This is good, thanks. It is useful to prevent notifications. What if I wanted to allow notifications?

You can leave "Block new requests asking to allow notifications" unselected. As more websites request to send notifications, they will be added to the list. If you want to allow notifications from a website you previously blocked, go to the notifications settings and set the website status to "Allow".

Thanks, there's only 1 website, on my list allowing notifications. I didn't see an option to add another, only to change the status of that one, or to remove all websites. There's a check box to block new requests for notification, which is not checked. If I denied notifications from a website, then changed my mind, how do I add it to the list?