回複：When asked what happened to the last job

If you were fired, I would call the ex-boss to check if he would commend as if you resigned youself (your ex-boss could vaguely word it as if you left on your own, if he wanted to do you a favor). If he is OK on that part, I would just tell in the interview that you want to move on to seek a job fit your background better.

If your ex-boss didn't agree, then find some reasonable excuses...