My dear clueless boy, Nokia 6303 Classic is not a Symbian phone, it's a Series 40 phone (actually Series 40 6th Edition) and that means it runs a proprietary Nokia OS on top of which there is a Java based (J2ME) application framework.

So, Symbian apps = No way! but Java apps = Yes! (but look for those claiming to be compatible with Series 40 6th Edition devices)