You appear to be using Firefox 143.0.1 on Windows 10 or 11 to post this thread. Note the Firefox UserAgent on Windows 10 and 11 shows NT 10.0 which ls used by both Windows 10 and 11 as to why it guesses you have Windows 10. If you are asking how to make Firefox the default web browser on Windows then see https://support.mozilla.org/en-US/kb/how-change-your-default-browser-firefox-windows