Marijuana is now legal in small amounts for recreational use. However, your company may have had a strict no-drug policy from the days before Proposition 64. If they fire you, is it a wrongful termination since you weren't breaking the law, or can the company let you go legally?
Though it may seem unfair to those who contest they've done nothing wrong, you can be fired for smoking marijuana. In fact, Proposition 64 specifically says that employer policies can remain and that employees can be let go for breaking them. The law does not preempt that policy.
In other states, like Colorado, lawsuits have cropped up after firings, but the courts have sided with the companies there, as well. One reason given is that marijuana is still illegal at the federal level.
So, what if you never actually smoked at work and you were not under the influence on the job? Technically, it doesn't matter. The company doesn't have to prove you were high on the job to fire you.
This is one way that the use of marijuana is typically different than other legal drugs, like alcohol. You can be fired for drinking on the job, of course, but you're not likely going to be fired for drinking at home. If you fail a marijuana drug test, though, even if you say you only smoked legally at home, Proposition 64 still allows for you to be fired.
It's always important to know how new laws are going to impact your rights, both as a citizen and an employee. Whenever you feel those rights have been violated, make sure you look into your legal options.
Source: Barron's Next, "Legal or Not, Smoking Weed Can Get You Fired," Crystal Kim, Jan. 26, 2017