> The databases reside on a Linux Mint server, but we back them up to

a Windows 7 system.   Both are running the same release of Firebird. 

  Are you sure you run Firebird 2.5 on Win7 (not 2.1) ?

Please, run following in command prompt (at firebird\bin folder) and show us results:

isql -user sysdba -pass masterkey
create database 'localhost:c:\temp\a.fdb';
show version;
drop database;
exit;

Regards,
Vlad