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.
1. Click the menu button 
in Firefox, then choose Options.
2. In the General pane in options window, click Make Default button.
3. Now the Windows Settings app will open. It will show the Choose default apps screen.
4. Scroll down the list and click the Microsoft Edge icon under Web browser.
5. In the Choose an app screen, click Firefox to set it as the default browser.
6. Now Firefox is your default browser.