Small. Fast. Reliable.
Choose any three.
This page provides a comparison (hopefully objective) between the embedded version Firebird and SQLite version 3.

Database File

Both database engines keep the entire database (all tables, indices, views, triggers, stored procedures, etc) in a single database file. At this time we have no hard information comparing the file sizes but we suspect they will be similar. SQLite will have a small advantage for the way in stores tables. Firebird will store indices more compactly, on the other hand.

The SQLite database file is cross-platform. Firebird databases cannot be copied between machines with differing byte orders or alignment restrictions. To move a Firebird database between platforms you have to back it up on the old platform then restore it on the new platform.

Engine Footprint

The complete SQLite library is about 230KiB statically linked. The size of Firebird is measured in megabytes.

Features

Both database engines support all the basics of SQL-92: tables, indices, views, triggers, etc. Support in Firebird is more complete. SQLite omits some features.

Speed

Rumors on the web indicate that the speed of both engines is comparible.

To Be Continued...