When you install Firefox after the fresh installation of Windows 10 you can have the option to set Firefox as default browser.
But when you upgrade to Windows 10 from Windows 7 or Windows 8, the upgrade process may change your default browser to Microsoft Edge. Even though your default browser is Firefox in old OS. You have to change it back again in Windows 10.
Do these easy steps to change the default browser back to Firefox.
5. In the Choose an app screen, click Firefox to set it as the default browser.
6. Now Firefox is your default browser.