Is the FF (Fuel Flow) the fuel consumption index?
The FF value can show either the fuel flow or the fuel used, depending on the position of a switch on the center forward panel. If it shows the fuel flow rate, then the value shown is in thousands of kilograms or pounds per hour per engine.


1 FUEL FLOW Switch (spring-loaded to RATE)
RATE – displays fuel flow to engine.
USED –

displays fuel used since last reset
after 10 seconds, display automatically reverts to fuel flow.

RESET –

resets fuel used to zero
displays fuel used for 1 second, decreases to zero, then displays fuel flow.

2 Fuel Flow (FF) Readout (digital)
[Option - kgs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (kilograms per hour x 1000).
[Option - lbs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (pounds per hour x 1000).
3 Fuel Used Readout (digital)
Displayed (white) – when FUEL FLOW switch moved to USED or RESET.

(Boeing 737 NG FCOM 7.11.13 - Engines, APU - Over/Under-Displays)