MPL/LGPL/GPL tri-license

- These licences refers to 'modification' done on the licensed software's (SPS / x4m) codes.
- 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.

- In our case, we don't modified any of Mozilla/Firefox codes, as well as x4m or SPS codes. All our codes are in separate files, which will be launch using SPS/x4m. Therefor our code is proprietary.It is like the example mentioned above.

- Does the GPL require that source code of modified versions be posted to the public?
The GPL does not require you to release your modified version. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.

But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL.

Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.