Examine source code of TST-DisableMiddleClick

{
  "applications": {
    "gecko": {
      "id": "tst-disable-middleclick@example.com",
      "strict_min_version": "57.0a1"
    }
  },
  "manifest_version": 2,
  "name": "TST-DisableMiddleClick",
  "version": "1.0.0",
  "description": "Extends TreeStyleTab to disable middle click for close tabs",
  "background": {
    "scripts": [
      "disableMiddleClick.js"
    ]
  },
  "permissions": [
    "sessions"
  ]
}