What is this thing on a FDX MD11 (MD10?)?
It's the infrared sensor for the Enhanced Flight Vision System (EFVS).
From FedEx Newsroom June 23, 2008:

FedEx Express is the first major commercial carrier in the airline
industry to receive a Supplemental Type Certificate from the FAA for
the advanced system, authorizing its installation in the company’s
fleet of Boeing MD-10 freighters. The company’s goal for the system is
to improve the level of flight safety by increasing visibility of
pilots during adverse weather conditions and darkness.
A unique application of Honeywell International “Head Up Display”
(HUD) technology combined with infrared “Enhanced Flight Vision
System” (EFVS) technology of Elbit Systems of America Commercial
Aviation-Kollsman Business Unit, positions the system as the leader in
the avionic visual technologies market.
The Honeywell HUD interfaces with aircraft navigational and flight
data systems in presenting a high resolution liquid crystal display of
critical flight guidance information. This is overlaid with real-time
EFVS infrared video of the outside world that is displayed in an
overhead unit in the captain’s forward field of view using
state-of-the-art HUD “combiner” technology. Elbit Systems Ltd.,
Electro-Optics-Elop manufactures the combiner glass.

The combined image appears like this:

Source: FedEx