When I first opened Firefox after installing, I configured all the settings for myself. And I unchecked >Always check if Firefox is your default browser. It checked it back on by itself and now I can't uncheck it! It's grayed out.
I suppose that is because you have to have a default browser. Well, strictly spaking you don't, I suppose, but some things are going to stop working.
It's not up to Firefox to scan your system for a list of browsers to offer you, so I suppose this is the easiest way.
As someone else said, just open up the browser you want to be default and check the option there.