At work we have a website which keeps trying to open pop-ups. I got tired of it and removed the notification. Now I want the notification back, but I can't find a setting for it anywhere.
You should see an icon the the location bar to indicate that pop-up are blocked.
You see a pop-up block icon on the location bar that allows to make changes.
If you click that icon then the drop down list has a checkbox "Don't show info bar when pop-ups are blocked"
You can look at this pref on the about:config page and reset them via the right-click context menu:
Info bar at the top: privacy.popups.showBrowserMessage