In some cases, the firebird database may be blocked and not opened simultaneously with another firebird version. In order to solve the problem,at the folder:
C:\Program Files (x86)\Firebird\Firebird_2_5\bin
Execute the commands:
gfix.exe -shut full -force 0 -user SYSDBA -pas masterkey "database.fdb"
gfix.exe -online "database.fdb" -user SYSDBA -password masterkey