Seems like there is no normal way of doing this, it's requested and known: https://connect.mozilla.org/t5/ideas/eyedropper-tool-shortcut/idi-p/137228 and https://support.mozilla.org/en-US/questions/1446238
One suggestion from these threads is to add the "Developer" button from "Customize Toolbar...". It's not a direct button for the eyedropper, but at least its only 2 clicks away now, instead using the main menu.