Avoid support scams. We will never ask you to call or text a phone number or share personal information. Please report suspicious activity using the “Report Abuse” option.
See:
*https://support.mozilla.org/en-US/kb/how-make-web-links-open-firefox-default
*https://support.mozilla.org/en-US/kb/how-change-your-default-browser-windows-10
Problems with specific programs:
*https://support.mozilla.org/en-US/kb/setting-firefox-default-browser-does-not-work