I have the same problem. The package installed in my computer is: 59.0.1+build1-0ubuntu0.16.04.1
This package puts icons of several sizes in: /usr/lib/firefox/browser/chrome/icons/default
but it only puts one icon in the directory /usr/share/icons/hicolor
namely in: /usr/share/icons/hicolor/128x128/apps/firefox.png
I have the same problem. The package installed in my computer is: 59.0.1+ build1- 0ubuntu0. 16.04.1
This package puts icons of several sizes in: firefox/ browser/ chrome/ icons/default
/usr/lib/
but it only puts one icon in the directory icons/hicolor
/usr/share/
namely in: icons/hicolor/ 128x128/ apps/firefox. png
/usr/share/