I hate these kind of questions! Last week I learnt what a 'womb' is, so I thought that I now could handle almost any english sentence And then you turn up and present us this word...!
apparently, the exact volume of a firkin varied depending on the product. 8 gallons of ale, 9 gallons of beer, or 10.5 gallons of salmon. All of them were still half a kilderkin.