Nor need you mind the serial ordealFor beer and ale a firkin is equal to nine imperial gallons or a quarter of a barrel (40.91481 litres). Casks in this size (themselves called firkins) are the most common container for cask ale.
Of being watched from forty cellar holes
As if by eye pairs out of forty firkins.
— Robert Frost, "Directive"
For wine the firkin had a larger size, namely a third of a tun. A tun being 210 gallons in the UK and 252 fluid gallons in the US, thus a wine firkin is about 318 L (318.226 or 317.975). It is also called tertian or, preferably, puncheon (in the US also shortened to pon).
The term firkin is currently used to refer to antique wooden buckets, usually with wood handle and lid, about 10 inches (250 mm) high and 10 inches in diameter (about 10L or 2-3 dry gallons in capacity), formerly used to store sugar and other items.
The firkin (a firkin of water) is the base unit of mass in the humorous FFF System.