Why?

I left after 5 month voluntarily, I wasn't fired. I performed well and they wanted me to stay, 5 months was just enough to understand what kind of company is it.

5 months sound about just right to get past the honeymoon phase with a new company and see what's swept under the rug, and seems fine given he/she wasn't fired.

They didn't say they were fired after 5 months. I left Amazon after 10 months. It had nothing to do with underperformance, it had to do with the working conditions within the team I was on. My manager was very happy with how I worked, and since leaving has emailed me to come and work for him on a different team.