Some sort of valve controller I think.
I have a 1996 Mercury villager there is a small
circular piece of equipment on the firewall that has
a vacuum hose coming out of it but I cannot figure
out where it is connecting to it does not hook into
the EGR valve system so it seems. It seems to be
some sort of vacuum device.
Sold mine, so I can't look. Could be to the EVAP system for the gas, in front of the tank under that shield. I replaced a valve down there, vacuum might come from that to the firewall.