There's not much we can say to help you.
You can always spin it in your favor and hope they don't go digging too hard:
Interviewer: So why did you quit Company XYZ?
You: It was a great place, and I enjoyed my time there, I really learned a lot. However, I found that I wasn't really a good fit for the company culture.
However, let's be real: if they know you were making twice as much they're going to be suspicious.
Your best bet is not to reveal that information, advice I would give anyone going into an interview, not only yourself. I, for example, personally make a point to refuse to state my current salary - only my salary goals. I hope that you can get away with doing the same where you're from, and that you can ask your former employer to withhold salary information as being personal data. If you can't, then you may wish to tell a half truth:
Interviewer: So why did you quit Company XYZ?
You: To be truthful, the reason I left company XYZ is because I was terminated for cause. The fault was entirely my own: I used the company laptop for personal entertainment, and it was a violation of company policy, even thought it happened on my own time. I learned a lot from this incident, and ... (you tell them how sorry you are, and how much more mature you are now)
In this situation it's very important to:
A) Downplay your offense (make it more like you were playing video games than doing .. other things)
B) Thoroughly explain how you've "learned your lesson", and would never do anything like that again
If you're at all suspicious about what the company might say if contacted for comment you could potentially ask a friend to call, pretending to be a recruiter. Some people do this, some people think it's silly. You decide if it makes sense for you.