Subject Firebird 2.5.3 & 2.5.4 allow connection to a restoring database.
Author John Price
Just discovered, when restoring a database using version 2.5.3 or 2.5.4 Firebird (superserver) will allow non SYSDBA users to connect to the database during a RESTORE even though the database is in single user maintenance mode. 
Version 2.5.2 will not allow the connection, says database is shutdown.

Win 8.1 64bit