I had this problem:
E: https://packages.mozilla.org/apt mozilla InRelease is not (yet) available (The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C0BA5CE6DC6315A3)
After many attempts it seems solved removing (renaming) the file .org.kde.neon.packages.mozilla.org.sources in /etc/apt/sources.list.d/