Show that you have Firefox installed in the add / remove applications(appwiz.cpl from run window). Let me boot windows and see what your options are.  ;-/

Here's what I see in Windows 11. see screenshots

Screenshot is here.I really can't see Firefox appearing in any of my default browser settings.

Show that you have Firefox installed in the add / remove applications(appwiz.cpl from run window). Let me boot windows and see what your options are.  ;-/

Screenshot is here.I really can't see Firefox appearing in any of my default browser settings.Thanks for your reply.

Show your default apps section.

Show your default apps section.

Screenshot is here.As you can see,the Firefox keeps showing that it's not the default browser and there is also not any Firefox option in my settings.What can I do，thanks for your reply.

What happens when you click the "Make Default" button. see screenshot Also try this... Install a different version of Firefox(Beta, Dev, or Nightly) and make it default. Make sure to install in a different directory than your current install. Then try again to make your regular version default.

What happens when you click the "Make Default" button. see screenshot Also try this... Install a different version of Firefox(Beta, Dev, or Nightly) and make it default. Make sure to install in a different directory than your current install. Then try again to make your regular version default.

When I click the "Make Default", it immediately jumps to the settings screen like I posted before, Then I can't make any relevant settings. And What does your last sentence mean? Are you telling me to install a different version of Firefox? And if so, will my existing data be lost?Thank you for your patience.

Different version and make sure to install in a path like something/Dev or /Beta or /Nightly. This will not impact your current install. You can then see if the new version shows in the default apps section.

Eventually, you may need to just re-install the same version as your current installed Firefox.