Techdows published a story that the Firefox browser can now be set as default PDF Reader in Windows 10.

Firefox can now be set as default PDF Reader in Windows 10

starting Firefox 78, the built-in PDF Viewer in Firefox (PDF.js) can be set as default PDF Reader on Windows. Mozilla has made the necessary changes to browser installer to register Firefox as a PDF handler on windows 10. Like Chrome and Microsoft Edge, Mozilla Firefox also comes with PDF Viewer built-in that displays PDF files inside the browser

Firefox can now be set as default PDF Reader in Windows 10