> I'am not quite sure - does Firebird 1.5.3 trim varchars when sending them to
> the client ?

Yes, read release notes:

Trimming of Varchar fields for remote protocols
Work was resumed and completed on this tricky feature for the 1.5 client and varchars now cross the
wire right-trimmed to actual length plus two bytes.
NOTE As it is the client that requests the server to trim varchars, the Firebird 1.5 client (fbclient.dll or
libfbclient.so) will trim, even if connected to a pre-1.5 server version. If you use an old client, you will
not get trimming, even if you are connected to a 1.5 or later server.