My boss threatened to fire me because I usually leave at 6pm and don't stay until 8/9 like other employees. How should I handle this?

You need to look at your employment contract. It will clearly state how many hours you are required to work that week. You’ll also need to examine any overtime agreements. If you’re fulfilling your hours then you’re perfectly within your rights to claim unfair dismissal if they do fire you, though with the way they’re treating you it could be a quick and easy way out with a free pay check for a while as you hunt for other jobs. It might also be worth discussing your hours of employment contract with your colleagues. Perhaps their contract is longer than yours and your boss needs to be reminded of this. You’ve got to know your worth, the job market, and what other companies are willing to pay for the same hours. Is the exploitation worth it? Or perhaps you’ve got it all wrong and you really need to put in that overtime.