How to rename a database table?

This is not possible. You have two options:

1. Create a new table with a different name. You can use your favorite administration tool to extract DDL statement for the old table and just replace the table name. For example, FlameRobin has a command that will drop and recreate all the related objects like the stored procedures, triggers, constraints, etc. After you create a new table, copy the data using a statement like this:

insert into newtable
select * from oldtable;


2. Create a view that is identical to your old table. In fact, you get to keep the table, and use the view as a kind of an alias:

create view newname (columns...)
as
select columns...
from oldtable;

Such plain VIEWS even allow DML statement like UPDATE, INSERT and DELETE and will change the records in the underlying table. Make sure you test this well because different Firebird versions may behave differently regarding the automatic behavior of such views.