It's more than just FOSS users. It's "The Internet" in general. At least two of the modding scenes I've been in have had multiple developers (and artists and translators) just quit due to their users aggressively complaining about the stuff they give away for free.
Of course, it doesn't get that much better when people have to pay for things -- ask customer service representatives how much toxicity they see from unsatisfied customers.
Again, check local laws, but in general you can be fired for cause (meaning no unemployment insurance eligibility) for violating company policy. So "legally" might be wrong but "had to clock out (if you want to keep working here)" might be accurate.