A San Francisco start-up released an early test version of its open-source Songbird music software, with which it ultimately hopes to undermine the dominance of Apple Computer’s iTunes. Pioneers of the Inevitable is hoping to create “the Firefox of MP3,” a reference to the open source Web browser that has eaten into Microsoft’s Internet Explorer market share. The company, which is led by digital music veteran Rob Lord, has cautioned the open-source community not to expect a full-featured music player from this first release, which they are calling only a “proof of concept” for now.