>
> 1- after some search , it seems that the recommended linux
> distribution for a web server is centos 7, then ubuntu .
>
> any feedbacks , recommendations, ideas on this please , and how it
> behave with firebird ?

I'm using firebird on debian for years - without any problems.
Presumably it does not really matter for firebird which distribution is
used.

> 2 - i am not really  a linux user , what is the best place/directory 
> to put the fdb databases ?

You can place them according to some standard documents as mentioned
before. But, it is also ok to place them in an Directory /db or
/usr/local/fbdata.

> 3 - i have several databases for differents custumers (20 / 30 or
> more), can a single firebird server handle many databases correctly ?

This number of databases does not cause problems. >ou should think about
spreading to multiple servers once the workload on your server does
raise and an single machine is not enough any more.

Elmar