> Apparently, it's possible with FireBird to set the port number in the
> connexion string. Is that right?

Correct. For example:

MYSERVER/3040:c:\data\test.fdb

Where MYSERVER is the host and 3040 the port MYSERVER is listening on.

> If so, starting from which version.

Firebird 1.0

> Also, can a FireBird 1.0 or 1.5 client machine communicate with an IB
> 6.0 server? Because I wouldn't mind upgrading the client machines if
> necessary, but I wouldn't like to touch the server.

The version of the client and server should always match, because
mismatched versions aren't tested. Plain and simple. Though, AFAIR
some peoples have mentioned that they are using the Firebird 1.0
client library with a IB 6 server. But, it's simply not recommended.
Don't know about the FB 1.5 client library and IB 6, and I'm not
sure that the specified port in the client connect string will work
against an IB 6 server.

--
Best Regards,
Thomas Steinmaurer

The IB LogManager Product Family
Logging/Auditing Suite for InterBase and Firebird
http://www.iblogmanager.com