Examine source code of re:calendar

{
  "manifest_version": 2,
  "name": "re:calendar",
  "version": "1.0.2resigned1",
  "description": "Add re:publica 2018 events to your calendar with an ical file",
  "applications": {
    "gecko": {
      "id": "firefox-recal@kroener.io"
    }
  },
  "icons": {
    "18": "icons/ic_alarm_add_black_18.png",
    "24": "icons/ic_alarm_add_black_24.png",
    "48": "icons/ic_alarm_add_black_48.png"
  },
  "background": {
    "scripts": [
      "libs/ics.js",
      "background.js"
    ]
  },
  "page_action": {
    "default_icon": "icons/ic_alarm_add_black.svg",
    "browser_style": true,
    "default_title": "Add event to calendar"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.re-publica.com/*"
      ],
      "js": [
        "recal.js"
      ]
    }
  ],
  "permissions": [
    "*://*.re-publica.com/*",
    "activeTab",
    "tabs"
  ]
}