It's so annoying that Windows 10>Default Browser won't accept Firefox as the default browser, so this is what I did:

  *  Go to Settings -> System -> Default Apps, scroll down to near the bottom.
  *  Click to choose Set default by app.
   * Click to choose 'Firefox', then click Choose defaults for this program.
   * Click Select all, assuming you are a diehard fan of Firefox like I am.
   * Save, and you're done!

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.