It is possible for you to compile and package the source code standalone or bundled with proprietory code as part of a larger works and sell it for profit. You may want to do this to provide product warranty. But, you must tell the licensee that the source code to EEA-owned part of the work is available and that EEA nor any contributor accepts any liability for the malfunction of the code. (MPL Section 3.6)
Any modifications you make to MPL covered code is covered by MPL with you in the role as contributor and you must make the modification available free of charge (MPL 1.1, Section 2.2)
Coverage is on source file level. Meaning: If you add a source file to the codebase, that file is owned by you, and you decide what license to use. But it must be possible to compile the product without that source file. If you modify a file already in the codebase the file continues to be under MPL, and you must make the file available as open source
[Open source software] returns control to the customer. You can see the code, change it, learn from it. Bugs are found and fixed quickly. And when customers are unhappy with one vendor, they can choose another without overhauling their entire infrastructure. No more technology lock-in. No more monopolies.
Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in.
In many organizations, developers have been using open source tools for managing code revisions and tracking bugs for many years. Now, they're adding communication tools, such as wikis, blogs, and forums, and a host of lifecycle management tools, such as continuous integration, lab management, and release tools, to create a richer ALM platform in the context of a community environment.