Just in case, reboot. It often help under MacOsX.

you should have after :

$ ps aux | grep fb
root 99059 0,0 0,0 2477544 540 ?? S 11:43
0:00.00 /Library/Frameworks/Firebird.framework/Resources/bin/fbguard
-daemon -forever
root 99060 0,0 0,0 2485856 5088 ?? S 11:43
0:00.02
/Library/Frameworks/Firebird.framework/Resources/English.lproj/var/bin/fbserver


and you can put in your path :
/Library/Frameworks/Firebird.framework/Resources/bin/

and then a :
gsec -user sysdba -password masterkey

should be enough