One of the things I was taught whilst studying engineering (in Australia) was if you, whilst acting in capacity as a professional engineer certify something knowing it is unsafe then you can be found personally liable.
Likewise if you knowingly observe anyone else in your company breaching safety/regulatory guidelines then as a professionally certified engineer you have a legal responsibility around ethical disclosure.
See: http://www.professionalengineers.org.au/rpeng/ethical-commit...
I do not know how things work in the US but in Australia these rights are protected by law. The company legally can not fire an engineer in this situation.