Try this : https://blog.mozilla.org/futurereleases/2015/07/29/firefox-for-windows-10-how-to-restore-or-choose-firefox-as-your-default-browser/

Think would need to reboot after but only if still does not appear.

Please let us know if this solved your issue or if need further assistance.

When i reach the page "This will open the Choose an app screen. Click Firefox in the list to set it as the default browser." only Edge and Internet explorer are shown on the list, not Firefox.

Please re-read the URL as you missed : The Windows Settings app will open with the Choose default apps screen. Scroll down and click the entry under Web browser. The Web browser icon will say either “Microsoft Edge” or “Choose your default browser”. It may not be intuitive, but you need to click on the Microsoft Edge logo to open the window that will let you choose another Web browser as your default.

Please let us know if this solved your issue or if need further assistance.