Currently there seems to be NO option to disable automatic desktop shortcut creation after first installation.
There should be a proper way to disable this obnoxious behavior, which happens with every update after the first and without any apparent solution to disable desktop shortcut creation.
e.g. https://www.reddit.com/r/firefox/comments/1pigq6w/how_do_i_stop_the_firefox_desktop_icon_from/, https://www.reddit.com/r/sysadmin/comments/164yp81/firefox_creating_desktop_shortcut_after_update/