When I read the report, without delay I fired off a letter to the editor.
If you fire off a shot, you send a bullet or other missile from a gun. A gunman fired off a volley of shots into the air an illustration of a guy firing a huge cannon off into the distance
If you fire off a letter, question, or remark, you send or say it very quickly, often as part of a series. He immediately fired off an angry letter to his ministry colleagues