You can mix MPL and MIT licensed code in the same project.
What you can't do (without the permission of it's copyright holders) is remove the MPL license from the code. This means that if you or anyone else wants to distribute the combined source tree or binaries built from both the MIT and MPL parts of it it they must comply with both licenses.
Accepting code under more restrictive licenses into your project effectively changes the license terms for people who want to use the project as a whole. This is especially true if it's not possible to exclude the more-restricted code at build-time. As a project maintainer that is not a descision to be taken lightly and could well lead to your project getting forked.