Stop Safari from asking to be default browser

My default browser is currently Firefox. On those rare times when I do open Safari, it displays a message at the top of the Favorites page asking if I want to make Safari my default browser. How do I make this go away?

Edit 1: I am using High Sierra.

Safari stops asking when it detects it has once been set as the default web browser, so the trick is to set it as the default browser and then switch back the default browser to Firefox.

1. Go to System Preferences > General
2. Set the Default web browser to Safari
3. Quit System Preferences
4. Open System Preferences > General
5. Set the Default web browser to Firefox (or any other browser)

Safari will no longer ask to be the default browser until the next major macOS upgrade.

Without knowledge of your macOS version:

1. Open System settings
2. Choose General
3. Choose from the pop-up menu in the default-Browser section your preferred browser.