create a new file named firebird in /usr/local/bin with your editor of choice...
the very first thing in this file is the shebang line....it tells your shell which interpreter to use...remember it must be at the very beginning of the file
then...just insert what you've already posted
Code:
/opt/fire/MozillaFirebird
then make it executable by typing
chmod a+x /usr/local/bin/firebird
easy, ain't it ? ^^
happy new year (my head still is aching =P)