How 'viral' is the MPL? If I use MPLed code in my proprietary application, will I have to give all the source code away?

    The MPL has a limited amount of 'copyleft' - more copyleft than the BSD family of licenses, which have no copyleft at all, but less than the LGPL or the GPL. It is based around the definition of a 'Modification' in the license [1.9].

    What is a Modification? Any changes to MPLed files, or new files into which MPLed code has been copied, are Modifications and so fall under the MPL. New files containing only your code are not Modifications, and not covered by the MPL.

    Files which fall under the MPL because they are or contain Modifications must be made available as detailed in the license (or elsewhere in this FAQ.) Other files may be kept proprietary.

    One well-known example of this is the Netscape-branded browser. It contains (and Netscape makes source code available for) many files from the Mozilla project, which are under the MPL. But it also contains proprietary code, for example to integrate with the AOL Instant Messenger service.