Cannot change default browser.

I've had my MacBook Pro for years and never had a problem with Safari being my default browser. Firefox did an update a few days ago and now it appears to be my DB. I tried to change it back to Safari from Safari Preferences and also from my Apple System Preferences. I can't do either. Under System Preferences there is nothing listed in the drop down menu, and under Safari preferences it just says "Safari is not your default browser" with a white box that says "Set Default". When I click that, nothing happens. Can someone help?