install firefox developer edition de alongside standard normal firefox in ubuntu
# download firefox archive from Mozilla Firefox Developer webpage: https://www.mozilla.org/en-US/firefox/developer/
# Extract it with file-roller
# rename firefox to firefox_dev
# move to /opt/
sudo cp -R firefox_dev/ /opt/
# create Desktop file:
nano ~/.local/share/applications/firefox_dev.desktop
# and add these lines:
[Desktop Entry]
Name=Firefox Developer
GenericName=Firefox Developer Edition
Exec=/opt/firefox_dev/firefox %u
Terminal=false
Icon=/opt/firefox_dev/browser/chrome/icons/default/default128.png
Type=Application
Categories=Application;Network;X-Developer;
Comment=Firefox Developer Edition Web Browser.
StartupWMClass=Firefox Developer Edition