https://linuxconfig.org/how-to-install-firefox-quantum-on-debian-stretc…

isto acabou por dar chatice!
novo caminho:

sempre como root.

apt remove firefox
apt edit-sources

remover os repositórios do sid

nano /etc/apt/preferences

comentar tudo o que tenha que ver com unstable

apt update

descarregar o quantum de https://www.mozilla.org/pt-PT/firefox/

mv Transferências/firefox-59.0.2.tar.bz2 /opt
cd /opt
tar jxf firefox-59.0.2.tar.bz2
chown -Rv xana:xana firefox

criar o ficheiro .desktop

touch /usr/share/applications/firefox-quantum.desktop

e copiar para ele o seguinte:
cat >>/usr/share/applications/firefox-quantum.desktop<< EOF
[Desktop Entry]
Name=Firefox ''Quantum''
Comment=Web Browser
GenericName=Web Browser
X-GNOME-FullName=Firefox ''Quantum'' Web Browser
Exec=/opt/firefox/firefox %u
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=/opt/firefox/browser/chrome/icons/default/default128.png
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

http://wiki.debian.org/Firefox
https://florent.poinsaut.fr/2018/03/02/installer-firefox-stable-debian-…
https://wiki.debian.org/HOWTO/DefaultWebBrowser