I doubt whether changing the registry is the answer. I want a TOGGLE, to be able to go QUICKLY between one or other as default.
My Firefox is quite old and so not suitable for many 'modern applications', that is why I want to be able to set Edge as a default.
But there are situations where my old Firefox can do what modern Edge can NOT, and it is required to be called repeatedly, that is why I want it as a DEFAULT until that task is finished, and then I'll go back to Edge. I want that going back and forth to be as quick as a click.