Open the StopTheMadness app.
In the StopTheMadness app, select "Install Firefox Add-on…".
Drag the StopTheMadness icon out of the "Install in Firefox" window and into the Firefox address bar.
Hover until the Firefox app becomes active, then drop the StopTheMadness icon into the Firefox address bar.
When prompted in Firefox, select "Add" to add StopTheMadness to Firefox.
You can select whether or not to allow StopTheMadness to run in private windows in Firefox.
StopTheMadness is now installed in Firefox. You can select "Add-ons" from the "Tools" menu to show StopTheMadness in Firefox.
You can change the website options in the Preferences for StopTheMadness, or by simply clicking the StopTheMadness icon in the toolbar. See Firefox Support for more information.
Some (but not all) Mac App Store updates may include updates to the Firefox add-on. When you open the Mac app after an update, it will inform you if an update is also available for the Firefox add-on.