Copy the firefox icon file.
cp /usr/share/applications/firefox.desktop \
/usr/share/applications/firefox-dev.desktop
In the new file edit 4 lines, using the full path for the executable and icon. The icon shown below is a blue colored version of the firefox icon.
[Desktop Entry]
...
Exec=/opt/firefox-dev/firefox
Icon=/opt/firefox-dev/browser/chrome/icons/default/default128.png
...
[Desktop Action new-window]
...
Exec=/opt/firefox-dev/firefox -new-window
...
[Desktop Action new-private-window]
...
Exec=/opt/firefox-dev/firefox -private-window
Then follow the instructions here to put the new icon file in the gnome registry.
sudo apt install dconf-editor
dconf-editor
Navigate to org.gnome.shell favorite-apps
and add in your new icon
[ ..., 'firefox.desktop', 'firefox-dev.desktop', ... ]
It may necessary to log out and log back in before using the icon.