>>107731208
Make sure there are no newlines or anything after the second EOF. Another way to do this is just to open an editor with root privileges and create /etc/apt/sources.list.d/mozilla.sources with the content
Types: deb
URIs: https://packages.mozilla.org/apt
Suites: mozilla
Components: main
Signed-By: /etc/apt/keyrings/packages.mozilla.org.asc