Henri Yandell and I have been having a civilized conversation about mixing Apache and MPL software. He permits me to share parts of it.
I modified Henri's definitions of "mix" and "appears with" by adding the underlined phrase about derivative works. These are now polite phrases instead of "ground beef" or "pee in a pool":
Mix and create a derivative work: When MPL source is added to existing Apache 2.0 source (the same file) and creates a derivative work such that the Apache 2.0 source must now be licensed under MPL.
Appears with: When MPL source is place beside existing Apache 2.0 source files, such that the Apache 2.0 source does not need to be licensed under the MPL.
You also have to create a derivative work. That is much rarer than you imagine. When you are merely mixing FOSS code for functional convenience (into the same or different files) and it has nothing to do with the copyrightable aspects of the works (e.g., DRM protection for the binary, packaging it into one TAR file, making separate programs convenient to install), then you have almost certainly not created a derivative work. That is not worth a $35 copyright application fee since any competent programmer in the world can replace that easily without infringement. For free. The only sour spot is that you are required by almost EVERY FOSS license to post notices of what you have mixed.
This is also why I believe that the GPL static linking doctrine is ridiculous. You also have to create a derivative work!
The two paragraphs above ought to be reviewed and approved by real attorneys working for real companies. I'm tired of talented programmers expounding otherwise on this legal issue. But if correct, it makes the combining of FOSS software generally trivial.
It does mean that you may more often ask your lawyer the following question, "Am I creating a derivative work?" rather than "Can I mix or appear with?"
If we're prepared to mix MPL inside Apache code, such that it can't be reversed, I think Apache should move to using MPL 2.0 as it's default license.
I have never suggested that and would never.
The Apache License is an excellent FOSS contributor and distribution license for lots of projects. I have myself written such FOSS licenses (e.g., AFL 3.0, in our Category A) that are fully compatible with and equally generous as ALv2. Our Apache projects currently understand third party code
and we don't randomly start mixing it up in our releases.
This is a GOOD THING. Nothing needs to change but our Third Party Licensing Policy. We just need to stop projects here from being much worried about mixing FOSS code and making it appear together, or at least to understand the legal definitions.
Only when an Apache contributor creates a derivative work of an MPL work (as perhaps by mixing it intimately and profoundly with ALv2 software) must the mixed result be under the MPL. Nothing wrong with that. Also that's free for anyone to do or undo.