Install firefox official stable release on debian stretch and consorts

Firefox upstream installer

#!/bin/sh
#Install firefox official stable release on debian stretch and consorts
set -e
TMP="$(mktemp -d)"
install_base="/usr/local"
echo "downloading latest firefox release..."
wget -O "$TMP/FirefoxSetup.tar.bz2" "https://download.mozilla.org/?product=firefox-latest-ssl&os=linux64&lang=fr"
echo "removing old firefox binaries"
test -d "$install_base/lib/firefox" && rm -rf "$install_base/lib/firefox"
echo "extracting..."
tar -C "$install_base/lib" -xjf "$TMP/FirefoxSetup.tar.bz2"
rm -rf "$TMP"
echo "Creating executable..."
ln -f -s "$install_base/lib/firefox/firefox-bin" "$install_base/bin/firefox"
echo "Installing desktop entry..."
mkdir -p "$install_base/share/applications"
cat > "$install_base/share/applications/firefox.desktop" << EOF
[Desktop Entry]
Name=Firefox
Comment=Browse the World Wide Web
GenericName=Web Browser
X-GNOME-FullName=Firefox Upstream Browser
Exec=/usr/local/lib/firefox/firefox-bin %u
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=firefox
Categories=Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/vnd.mozilla.xul+xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;
StartupWMClass=Firefox
StartupNotify=true
EOF

echo "Installing icons"
#some dirs might not exist
icons_base="$install_base/share/icons/hicolor"
#usage : link_icon <src> <size>
copy_icon(){
	local src="$1"
	local size="$2"
	local destdir="$icons_base/${size}x${size}/apps"
	test -d "$destdir" || mkdir -p "$destdir"
	echo "\tcopying $destdir/firefox.png"
	test -f "$destdir/firefox.png" || cp "$src" "$destdir/firefox.png"
}
#default128.png  default16.png  default32.png  default48.png  default64.png
(
	set -e
	cd "$install_base/lib/firefox"
	for icon in browser/chrome/icons/default/default*.png ; do
		size="$(echo -n "$icon" | sed -e 's/^.*default\([[:digit:]]\+\).png/\1/')" #get size
		copy_icon "$(pwd)/$icon" "$size"
	done
)

update-alternatives --install /usr/bin/x-www-browser x-www-browser "$install_base/bin/firefox" 80
update-alternatives --set x-www-browser /usr/local/bin/firefox

Just to be clear : did you put the firefox symlink in /urs/local/bin on purpose? Because I'm afraid that /usr/bin/firefox symlink to firefox-esr will override it

It should not. /usr/local/bin is generally before /usr/bin in PATH so the local version will be used. It's even used as the default browser handler because it runs update-alternatives at the end.