/etc/apt/sources.list
deb http://ftp.us.debian.org/debian/ sid main

/etc/apt/preferences
Package: *
Pin: release a=stable
Pin-Priority: 1000

Package: *
Pin: release a=unstable
Pin-Priority: 2

Package: firefox
Pin: release a=unstable
Pin-Priority: 1001

Package: libfontconfig1
Pin: release a=unstable
Pin-Priority: 1001

Package: fontconfig-config
Pin: release a=unstable
Pin-Priority: 1001

Package: libnss3
Pin: release a=unstable
Pin-Priority: 1001

apt update
apt install -t sid firefox