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.
Set your default browser to another one, then it should be clickable again