The easy way to enable installation of packages, adapted from Mozilla's guide. Files should be saved to:
/etc/apt/preferences.d/mozilla
/etc/apt/sources.list.d/mozilla.sources
The PGP key is Mozilla's, which you can compare to the one embedded in the .sources
file.
The fingerprint should match the required value 35BAA0B33E9EB396F59CA838C0BA5CE6DC6315A3
(as per their guide), which can be viewed like so:
grep -zoP 'Signed-By:\s*\K.*(?:[\r\n]+ .*)+' /etc/apt/sources.list.d/mozilla.sources \
| sed 's/^ \(\.$\)\?//' \
| gpg --show-keys