Change firefox snap packages desktop icon
If you use a custom icon theme (for example numix-circle) under GNOME and want to change the icon for firefox.
Desktop icons for snaps are located at:
/var/lib/snapd/desktop/applications/
To change the firefox icon:
Copy the relevant .desktop file to ~/.local/share/applications/.
cp -f /var/lib/snapd/desktop/applications/firefox_firefox.desktop ~/.local/share/applications/
Edit the .desktop file and change Icon element.
nano ~/.local/share/applications/firefox_firefox.desktop
Change line below (or similar) from
Icon=/snap/firefox/3068/default256.png
to (for example)
Icon=mozilla-firefox.svg
Save and exit your editor.