I was unable to replicate this solution your way but had success eventually a similar way. Windows kept using Edge even after following your steps and guides, what eventually led to my success was copying the Firefox information from the UserChoiceLatest key into the User Choice key. this obviously made windows give me an error but it set browser information to default across windows and then when I went back through settings like a normal user and set everything to Firefox again it worked as normal, including Epic Games Launcher.