Disabling the "Enable notifications?" popup in Firefox

Go into about:config and set dom.webnotifications.enabled to false. This seems to work for me in FF 47.

Go into about:config and set dom.webnotifications.enabled to false

You can also disable this on a per-site basis by right-clicking somewhere in a blank spot of the site (or pressing Ctrl-I on Windows or Cmd-I on Mac) and then selecting "View Page Info" from the opening menu. This opens a window with a "Permissions" tab. Scroll down until you see "Receive Notifications", uncheck "Use Default" then select "Block."

As of Firefox 59, this is in Options

Open the Options window

On the left side of the screen, click "Privacy & Security"

Under the "Permissions" section, find "Notifications" and click the "Settings..." button

Check the box that says "Block new requests asking to allow notifications"