Uhm...
I've changed my code:

long s = System.currentTimeMillis();//<<<< moved here
PreparedStatement pstmt = conn.prepareStatement("insert into pics
(id_ref,pic_obj) values (?,?)");

pstmt.setLong(1,2);
pstmt.setBinaryStream(2,fis,fis.available());


pstmt.executeUpdate();
pstmt.close();
conn.commit();
long e = System.currentTimeMillis();
System.out.println("Time needed: "+(e-s));

Now the time measuring includes the creation of the preparedStatement
and all setXXX methods (which probably also produce some network
traffic, I assume).
The results are now:
interclient : 1622 ms
firebirdsql : 3965 ms

As you can see firebirdsql ~2.5 times slower.

Following your calculations with a 4 Mbits/s connection it should take
for you 67.04Mbits / (4 MBits/s) = 16.755 s (plus something for
protocol overheads) for interclient and ~42 s for firebirdsql.

Asynchronous calls?? I don't think I understand, but my code is as
simple as shown above. My application i single threaded, my FB
database runs on a Win2000 Pro workstation (PIII 800Mhz, 256 MB ram...
not a "monster").

Ciao
Luca