For Debian 9.0 run the following:
Keep in mind that the owner of the key may distribute updates, packages and repositories that your system will trust (more information).
echo 'deb http://download.opensuse.org/repositories/home:/hawkeye116477:/waterfox:/build-depends/Debian_9.0/ /' | sudo tee /etc/apt/sources.list.d/home:hawkeye116477:waterfox:build-depends.list
curl -fsSL https://download.opensuse.org/repositories/home:hawkeye116477:waterfox:build-depends/Debian_9.0/Release.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/home_hawkeye116477_waterfox_build-depends.gpg > /dev/null
sudo apt update
sudo apt install gcc