Click the Download for Mac link. The software will begin downloading. Click on the download icon in the upper-right corner of your browser after download completes. Open and run the file.
Software Installation Help - Firefox for Mac
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Run installation. Once complete, click Finish.
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You will be taken to this page after the installation. Click on the Night Shift icon to open the application launch. Then click Open Link.
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To start the software in the future, you just need to double click the icon on your desktop.