Notifications (a.k.a. OSD) is one of those UI features which seems fancy at first, but quickly gets in your face. Possibly originated on the Mac, the annoyance has now spread to Firefox, Fedora and Ubuntu as well. Here's how to quickly switch them off:
Enter about:config in the location bar. Search for this preference, and double click.
Fedora and Ubuntu
sudo mv /usr/share/dbus-1/services/org.freedesktop.Notifications.service /usr/share/dbus-1/services/org.freedesktop.Notifications.service.disabled