Greenskins will drink anything, much to the Dwarfs chagrin --In-game description
Mugs Firebrew is that rarest and most honored of barkeeps: a master brewer.
Having created scores of unique brews in his long and illustrious career, it is strange that only one bears his name. The recipe for Firebrew is a closely guarded secret; a secret not even shared with his own sons.
There are some Dwarfs who fear that, should Barak Varr fall, the recipe for Firebrew will be lost forever. Locating a barrel of the rare stuff is considered a worthy quest for young Dwarfs the world over, second only to the Bugman's pilgrimages. - In-game Description
There's a 'Floating Barrel' in the water in the Barak Varr area near the Greenskin Chapter 8 camp. Location is (53479,24748).