It all depends on your needs, but Firebird is very easy to "install" for, say, a simple application you could unzip the "embedded" kit in your application directory (more or less as if you were using, say, SQLite), giving you the option of keeping your program as 64 bit while the Delphi keeps its own 32 bit install.
Read the documentation to find how all this works and don't desperate

Though, of course, if you don't care whether your program is 32 or 64 bit, making it 32 bit might be the easiest solution, perhaps. Depends on how the Delphi program is using Firebird, whether a standard server or an embedded one.
ETA: Forgot to add -- If you want an overview of how Firebird works for a Lazarus application see the wiki page on
Firebird