Grab yourself the FB 1.5 RC1 here:
http://www.amsoftwaredesign.com/downloads/Firebird-
1.5.0.3233_RC1_win32_setup.EXE

Has built in ifnull,CASE statements,COALESCE and nice speed
increases over FB 1.x

Here is a Open source UDF that implements ifnull and NVL functions
from oracle here:
http://www.amsoftwaredesign.com/downloads/tudf.zip

The UDF compiles for win32 or Linux using Delphi or Kylix.
It may also work with free Pascal with some tweaking.

Has tons of other functions as well.

You have to change your way of thinking when moving from MS SQL
server to FB, but you can do it with spectacular results.