Here's the error message:
E: http://security.ubuntu.com/ubuntu bionic-security/main amd64 firefox amd64 62.0.3+build1-0ubuntu0.18.04.1 is not (yet) available (404 Not Found [IP: 2001:67c:1560:8001::14 80])
I was able to download Firefox directly from their website, but of course, it doesn't show up in the Apps Center. Today I cannot launch it by clicking on the executable.
Update: The firefox exe file now launches firefox... I moved the executable. My bad. I moved it back and now it works. Real question is, why can't I load the free version from the apps center? also, no other apps are showing up in the apps center...