I have this problem with Firebird 2.1, this is what I do:

1.- Create a connection to firebird and open it (It doesn't matter if
you use .NET or Delphi). Don't close it, leave it open.
2.- Check the memory of FBServer process.
3.- Create another connection, open it and close it many times, you
will see how the memory of FBServer grows every time you do it.
4.- Close the first connection you created in step one, you'll see how
the memory returns to its inicial state.

Now, this doesn't happens with FB 1.5, so what is the problem with FB 2.1?