It can be used in proprietary software. You shouldn't, because that's unethical, but the MPL isn't a strong copyleft. And even with strong copylefts, there are

It can be used in proprietary software. You shouldn't, because that's unethical, but the MPL isn't a strong copyleft. And even with strong copylefts, there are only three that I can think of that apply to software that won't end up on user devices.