> I'd like to see SQL statement to be processed.
> How can I see SQL statements to be processed in the Firebird server?
> Is there any log file?

No.

The only thing you can do is set up a SQL monitor on your client
side to watch SQL statements from your applications to the Firebird
server.

At the server side, there's no "easy" way to know what SQL statements
are being executed/arrive at the server.

Although this product might be interesting for you:
http://www.h-k.de/index2.php?var_content=ibmonitor&var_produkt=hk&var_lang=englisch

With regards,

Martijn Tonies
Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL
Server
Upscene Productions
http://www.upscene.com