It's still "unlawful" under federal law. My view: So your state makes it legal to use marijuana, and your boss fires you for doing so. Courts haven't been eager to help employees in that situation. Here's a recent example:
Colorado has an unusual statute that generally makes it an unfair and discriminatory labor practice to discharge an employee based on the employee's "lawful" outside-of-work activities.
Brandon Coats is a quadriplegic and has been confined to a wheelchair since he was a teenager. In 2009, he registered for and obtained a state-issued license to use medical marijuana to treat painful muscle spasms caused by his quadriplegia. Coats consumes medical marijuana at home, after work, and in accordance with his license and Colorado state law.
Coats worked for Dish Network, a company that randomly tests employees for drugs. Coats tested positive for THC, and Dish Network fired him.
Coats argued that his use of medical marijuana was "lawful" under state law, so he couldn't be fired for using it.
Dish Network argued that marijuana was "unlawful" under federal law, so the state statute did not protect him for being fired.
In Coats v. Dish Network (Colorado 06/15/2015), the Colorado Supreme Court unanimously sided with the employer, saying:
Nothing in the language of the statute limits the term "lawful" to state law. Instead, the term is used in its general, unrestricted sense, indicating that a "lawful" activity is that which complies with applicable "law," including state and federal law.
In most states this is not an issue because most states don't have Colorado's outside-of-work statute. If the boss wants employees to be pot-free, then so be it.