Today should be cheerful. :)
So, Firpo was a pianist, and composed many pieces, like 'Fuegos artificiales', but also part of the 'Cumparsita'! And other tangos like 'El amanecer' (listen to the version by di Sarli), 'Arganaraz' (Tanturi's version), or 'Alma de Bohemia' (I sent Laurenz+Podestá's version yesterday).
As for the piano, one could say that he brought piano into tango. Earlier tango groups were based on flute, guitar and violin. Partly this was because recording didn't work properly: when you had a piano that would be far louder than the rest, if you couldn't amplify individual instruments... And live concerts had the same problem, plus not all places had pianos. Bandoneon came into tango even later by the way!
Anyway, if you listen to the old Firpo you get a touch on the roots of tango. Back in the beginning I guess he was modern and experimental, but later he became more conservative. I like his style a lot actually, very happy and interesting.
You wouldn't hear this music in most milongas though, I think it's because the tangos resemble really old style, and we'd even find them hard to dance. His tangos are mostly pretty fast, with a milonga-type rhythm, hard to place. But it's nice to see where tangos came from.
Now, to recognize Firpo, one thing for sure would be the flute. I guess the piano playing is also quite particular.
Finally, a little anecdote: at some point in the 30s he lost a lot of money when he had stopped with tango, bought a farm and cattle, and the Paraná river flood destroyed everything. Later, through gambling, he lost the rest --- at which point he went back to tango to earn money again. :)
http://www.youtube.com/watch?v=A4fMy5UoeVk
http://www.youtube.com/watch?v=Ebh0_T-dmx8 (really early tango)