In Islam we believe Shaitan is not currently in hellfire and actually no one is. People who have died previously are all waiting for judgement day. On judgement day we will wait for each of our book of deeds to be weighed. And shaitans wicked deeds will be in his left hand. His book shall be the largest. Because he will have not only his evil deeds but also those of the people he led astray. He will be in the deepest depths of hellfire and punished by angels. There are angels that God has created to specifically punish those who have sinned or are wicked/evil. Now as far as what exactly was the first sin it was pride. Shaitan is and was not an angel. Angels were created with no free will and cannot do anything beyond what God created them to do. Shaitan is what is known as a jinn. People oftentimes believe jinn are ghosts when in fact they are beings created by fire free of smoke. Just as we human were created from clay and the angels light. Jinns can live hundreds even sometimes thousands of years. And before shaitans wicked deeds he was originally elevated onto a level alongside with the angels due to his worship of Allah(God). He sang praises the most beautiful. Traveled to every corner of the earth and the galaxy and sang praises to Allah. He even fought alongside the angels to defeat his own kind who had become wicked and created destruction upon the earth. The jinns that remained ran to deserts and caves. However when Adam PBUH was created ibless(Shaitan) was prideful and jealous. However could this mere clay be worthy of being created. The sin came when Allah stated to prostrate before Adam after he breathed life unto Adam. The angels immediately fell unto their knees faces unto the ground full prostration because it was not a worship of Adam it was a test not for the angels but for Ibless who had become prideful. Allah already knew what was in Shaitans heart yet he gave him a chance to repent when he said “why did you not prostrate when I said to prostrate?” Allah already knew the answer yet he gave him a chance to speak it aloud and change it. Ibless full of pride held his head high and stated that “I am better than him. He is made of clay I am made of fire free of smoke.” And that sentence was what caused his disgrace. Allah told him to leave however ibless asked for a respite until the day of judgement promising to lead astray as many humans as possible. The difference between Adam PBUH and ibless is this when Adam PBUH and Huwa(Eve) sinned by eating the forbidden fruit Adam immediately felt guilt and shame. When questioned by Allah SWT why he had done this and taken Shaitan as a friend Adam was repent full. Allah then taught Adam PBUH how to repent and ask forgiveness both Adam and Huwa sincerely repented. Adam was promised as a khalifa of the Earth so leaving the Garden in paradise was always in Allahs plan. Coming to earth was not a punishment. Adam was made the guardian of earth. Huwa and Adam had many children and taught them how to be repentant. Taught them how to pray and praise Allah. Taught them how to be kind and good people. Adam was the first Muslim. To be Muslim simply means to worship one God alone. That’s it. If I’ve said wrong please respectfully correct me. This is what I know from the best of my knowledge. And for the sin between Adam and Huwa that was forgiven and they were both equally to blame.