Jamieson, Fausset & Brown says, " 6. firkins--about seven and a half gallons in Jewish, or nine in Attic measure; each of these huge water jars, therefore, holding some twenty or more gallons,
for washings at such feasts ( Mar 7:4 ).
Mark 7:4 "And [when they come] from the market, except they wash, they eat not. And many other things there be, which they have received to hold, [as] the washing of cups, and pots, brasen vessels, and of tables."
Wonder why they would want to drink water that people washed their hands in?