Open the StopTheMadness app.
In the StopTheMadness app, press the Firefox button.
(If you have Firefox Developer Edition on your Mac, there will be a separate button for it in StopTheMadness.)
This opens a window in Firefox. Just click the link to install StopTheMadness for Firefox.
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.