Download the bin from the official page:
https://www.mozilla.org/de/firefox/developer/
sudo cp -rf firefox-86.0b9.tar.bz2 /opt
sudo tar xjf firefox-86.0b9.tar.bz2
sudo chown -R $USER /opt/firefox
nano ~/.bashrc
-> export PATH=/opt/firefox/firefox:$PATH
cat > ~/.local/share/applications/firefoxDeveloperEdition.desktop <<EOL
[Desktop Entry]
Encoding=UTF-8
Name=Firefox Developer Edition
Exec=/opt/firefox/firefox
Icon=/opt/firefox/browser/chrome/icons/default/default128.png
Terminal=false
Type=Application
Categories=Network;WebBrowser;Favorite;
MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp; X-Ayatana-Desktop-Shortcuts=NewWindow;NewIncognitos;
EOL