Playing with fire (literally) — handling fire (matches, lighters, etc.) in a playful or careless way.
子どもがマッチで火遊びをしているのを見つけた。
I found a child playing with matches.
古い納屋で火遊びがもとで火事が起きた。
A fire started in the old shed because of someone playing with fire.
キャンプ場では火遊びは厳禁だ。
Playing with fire is strictly forbidden at the campsite.
Figuratively, doing something dangerous or risky — behaving in a way that invites trouble (playing with fire).
あの会社との取引は火遊びだ。問題が起きる可能性が高い。
Doing business with that company is playing with fire. There's a high chance of problems.
ギャンブルで借金を取り返そうとするのは火遊びに等しい。
Trying to recover debts by gambling is like playing with fire.
法律ぎりぎりのやり方は火遊びだと考えた方がいい。
You should consider borderline-legal methods as playing with fire.
Flirting or having a casual/brief affair — messing around with someone emotionally or sexually.
彼は若い頃、よく火遊びをして女性関係で問題を起こした。
When he was young he often messed around and caused trouble in his relationships with women.
そんな軽い火遊びで人を傷つけてはいけない。
You shouldn't hurt people with such casual messing around.
彼女の噂を信じると、火遊びに巻き込まれるよ。
If you believe her rumors, you'll get drawn into flirtations/affairs.