The problem was in the manifest of my extension. I declared my content script as a background script.

So, instead of writing this :

"background": {
    "scripts": ["myscript.js"],
    "persistent": true
    },

You have to declare the script like this :

   "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["myscript.js"]
    }
  ]