Mike Schroepfer, the extremely well regarded VP Engineering at Mozilla, is now Facebook’s Director of Engineering.

He’ll be heading up Facebook Platform and the main product front end, he said by telephone this morning, although his exact scope of responsibility hasn’t been nailed down yet. He will report directly to CEO Mark Zuckerberg starting in September.

Schrep, as he is known, headed up the engineering team within Mozilla responsible for Firefox. Before that he was the CTO of Sun Microsystem’s data center automation division (responsible for the highly ambitious N1 project). In order to become a leading Internet platform, Facebook needs to inject more open-source DNA into its engineering ranks. Schrep should help it do that.

Since Mozilla won’t IPO, it could face more defections of top talent over time.