1. In the Menu bar at the top of the screen, click Firefox and select Preferences.Click the menu button and select Options.Preferences.Settings.
2. In the General panel, click Make Default….

   

   • If Firefox still isn't the default browser, see Setting Firefox as the default browser does not work - What to do.
3. Close the about:preferences page. Any changes you've made will automatically be saved.

1. In the Menu bar at the top of the screen, click Firefox and select Preferences.Click the menu button and select Options.Preferences.Settings.
2. In the General panel, click Make Default….

   

3. The Set Default Programs window will open.
4. In the Set Default Programs window, select Firefox from the list of programs on the left and click Set this program as default. Then click OK to close the window.

   

5. Close the about:preferences page. Any changes you've made will automatically be saved.

1. In the Menu bar at the top of the screen, click Firefox and select Preferences.Click the menu button and select Options.Preferences.Settings.
2. In the General panel, click Make Default….

   

3. The Windows Settings app will open with the Choose default apps screen.
4. Scroll down and click the entry under Web browser.

   

5. Click on Firefox in the dialog that opens with a list of available browsers.

   

6. Firefox is now listed as your default browser. Close the Settings window to save your changes.

Use the Settings app in Windows 10 to change your default browser

Alternatively, you can open the Settings App from the Windows 10 Start menu to set Firefox as the default browser:

1. Go to the Windows Start menu and click the Settings icon.
2. Click Apps, then choose Default Apps on the left pane.
3. Scroll down and click the entry under Web browser.

   

4. Click on Firefox in the dialog that opens with a list of available browsers.

   

5. Firefox is now listed as your default browser. Close the Settings window to save your changes.

Based on information from Default browser (mozillaZine KB)