recreate table t (a varchar(30), b varchar(30));

insert into t(a,  b) values('Заменить1', 'Заменили1');
insert into t(a,  b) values('Заменить2', 'Заменили2');
insert into t(a,  b) values('Заменить3', 'Заменили3');

commit;

set term ^;

create function replace_by_table(StrParam varchar(4000)) returns varchar(4000)
as
begin
  for select a, b
      from t
      as cursor c
  do
  begin
    StrParam = replace(StrParam, c.a, c.b);
  end
  return StrParam;
end^

set term ;^

select replace_by_table('Заменить1 , Заменить2 , Заменить3') as StrParam
from rdb$database