A person who genuinely and seriously calls out somebody's legitimate complaint as a first world problem as a means of trivialising them and their complaint, usually to obtain a smug sense of moral superiority because of how 'in touch' they are with the 'plight of the underprivileged'.
Dave: "Great, my air conditioner just died in the middle of a heatwave! This sucks."
Twat: "This is such a first world problem. At least you have a house and clothes, and perpetual access to the internet on your smartphone to make these status updates."
Dave: "Don't be such a first world twat."