Boss fired for smoking weed - can I get in trouble for not reporting it?

Yeah well on an ethical grounds he may have a case, I don't know if it is enough to fire you though. If your contract states that employment can be ended at any time for any reason (most of the time this is the case), then the employer doesn't even need to give a reason. In my opinion you should just say you weren't sure that it was weed or not and although you suspected it, you umed that your boss would not be doing illegal drugs in the workplace.