(sorry I took some time to come back to this) I’m not entirely sure the #5 of OSD would apply to the binary results (even though they are considered derivative work). GPL does restrict the license of the binary, but it seems MPLv2 allows licensing the binary differently as long as you don’t restrict the code further:

You may distribute such Executable Form under the terms of this License, or sublicense it under different terms, provided that the license for the Executable Form does not attempt to limit or alter the recipients’ rights in the Source Code Form under this License.

These cause don’t biphobia

How can Firefox not be open source if its sources are under the MPL2 ?
It has always been the case that Firefox is a trademark and you can’t distribute it under that name. However if the code is open source the project is too.

Or work together and do the job in half the time and enjoy the rest