First it seems to be for any kind of open source:

    Mission Partners

    The Mission Partners track is open to any free software / open source project
    that significantly furthers Mozilla’s mission to ensure the Internet is a
    global public resource, open and accessible to all.

But later on it specifies something that aligns quite well with Mozilla's principles:

    Activities which are not sufficient for an award on their own include:

       Writing open source software (notwithstanding manifesto principle #7)
       Building something cool with web technologies
       Translating open source software into another language

And it should be endorsed by someone well-known:

    The specific project activity should be endorsed (on the application form) by someone well-known [...]

Which... is quite hard and not just any kind of open source project. It seems you have to be high profile already to even opt for it, then do something along Mozilla's main principles.