> Till now, I was using FireBird 2.0.1 and from now on I will use FireBird 2.1.2. So I will tell my customers to install the new version of my application choosing to install FireBird again.

I hope that you also tell them to perform all necessary procedures to
upgrade their databases from 2.0 to 2.1, including fixing metadata
charset as described in migration guide...

> 1. Why differents versions of FireBird installs in diferent folders ..\FireBird_N_M?

Because they can co-exist and even work simultaneously.

> 2. Is it really necessary (or important) to deinstall the older version of FireBird? Why?

No. As I already told few Firebird servers can be installed (and even
work) at the same time.

SY, SD.