Although Firefox is my default browser, every time I ope it I get a mesaage that it is not my default.
Whenever I open Firefox, I get the message that it is not my default browser, but when I check my PC settings, it does show Firefox as my default browser. I run Windows 8.1. When I try to reset it in my Firefox options and I click on "Make Default", nothing happens. This just started suddenly and I have no idea why.
It's possible something is messing with your settings. Make Firefox your default browser
I'd suggest setting something else as your default, closing Firefox, then opening it again and setting Firefox as your default. That might clear the confusion