When I first moved to Allershausen I was sent a bill for about 30DM (€15) as I wasn't in the FFW.
In Bavaria men between 18 and 60 had to pay a Feuerschutzabgabe if they were not members of the FFW.
The European Court of Justice ruled against this in 1994 as women had not to pay.
See e.g.
this Focus arcticle from 1994.
The Grundsteuer (property tax) was increased afterwards to make good for the loss of the Feuerschutzabgabe.
Of course one, albeit minor, reason to join the FFW disappeared with the Feuerschutzabgabe.