Can't set Firefox as Default Browser in Windows 10

Earlier this year, I finally upgraded my computer from Windows 7 to Windows 10. All went well. I had been using Pale Moon as my default browser in Windows 7 and continued to do so for a while in Windows 10. But I finally decided that I wanted to use Firefox as my default browser. I went into Settings, Apps, Default Apps. I saw that Pale Moon was set as the default. To my dismay, I was unable to change that to Firefox. I was given a choice of Microsoft Edge (listed twice, Internet Explorer, Opera (a VERY old version that I probably should remove), and Check in Microsoft Store. Firefox was simply not listed. I then tried to set default by app. I was able to set Firefox for http, but not for https, nor for opening url links. It simply wasn't listed. Out of desperation, I even looked in the Microsoft Store, but of course it wasn't there. I really would like to use Firefox as my default. Is there some way to do this aside from the one that doesn't work?

Thanks in advance for help.