Reproduced with ISQL and .bat
With 10 concurrent connections running, at 20 hangs.
firebird.log is empty.

test.bat:
@echo off
for /L %%i in (1,1,10) do (
   for /L %%j in (1,1,20) do start isql -u SYSDBA -p masterke -i _input.sql -o _output.txt "192.168.10.10:d:\Program Files\Firebird\Firebird_2_5\examples\empbuild\employee.fdb"
   isql -u SYSDBA -p masterke -i _input.sql -o _output.txt "192.168.10.10:d:\Program Files\Firebird\Firebird_2_5\examples\empbuild\employee.fdb"
)


_input.sql:
with
nx as
(
  select 0 i from rdb$database union all select 1 from rdb$database union all
  select 2 from rdb$database union all select 3 from rdb$database union all
  select 4 from rdb$database union all select 5 from rdb$database
)
select count(*) from nx n5, nx n4, nx n3, nx n2, nx n1, nx n0 where n5.i+n4.i+n3.i = n2.i+n1.i+n0.i;
select count(*) from mon$attachments;
with
nx as
(
  select 0 i from rdb$database union all select 1 from rdb$database union all
  select 2 from rdb$database union all select 3 from rdb$database union all
  select 4 from rdb$database union all select 5 from rdb$database
)
select count(*) from nx n5, nx n4, nx n3, nx n2, nx n1, nx n0 where n5.i+n4.i+n3.i = n2.i+n1.i+n0.i;
exit;