Use following command to locate firefox installtion:
$locate firefox
To determine location of icon PNG file, just in case
It's currently located here:
/usr/share/icons/hicolor/48x48/apps/firefox.png
Right-click the browser icon in your VM choose properties, and then click the current icon, navigate to the following folder, and then click icon and choose the firefox.png icon.
/usr/share/icons/hicolor/48x48/apps/