Add repository to sources.list:

deb http://ftp.us.debian.org/debian/ sid main
deb-src http://ftp.us.debian.org/debian/ sid main

firefox.pref

# Pinning setup for firefox:
# ==========================
# 1. Review the files in /etc/apt/preferences.d:
#    Pin-Priority should be: unstable<testing<stable<Debian Security
# 2. Review and copy this file to /etc/apt/preferences.d
# 3. Update system, no new packages should be installed!!! 
#     # apt update
#     # apt upgrade  << No new packages, otherwise pinning is wrong. ABORT!!!
# 4. if all was ok above
#    # apt install -t sid firefox 
# Ref: https://linuxconfig.org/how-to-install-firefox-quantum-on-debian-stretch-linux
#
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

Ref: https://linuxconfig.org/how-to-install-firefox-quantum-on-debian-stretch-linux