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..._