Safari won't open; can't change default browser

Three days ago Safari began failing to open, at all. I've used Firefox to browse since then but when I try opening a link by clicking on it (as in an email) it defaults to Safari. I understand that the ability to change the browser default is now a part of Safari Preferences (?). Feels like a Catch 22. I've used Safari for years and like it, but am not great at fully understanding all aspects of my own computer. Any suggestions for how to either: change the browser default to Firefox, or make Safari work again?