launch.json

{
  "version": "0.2.0",
  "compounds": [
      {
        "name": "Attach/Launch",
        "configurations": ["Attach:Firefox", "SF"]
      }
    ],
  "configurations": [
    {
      "name": "Attach:Firefox",
      "type": "firefox",
      "request": "attach",
    },
    {
      "name": "SF",
      "type": "firefox",
      "request": "launch",
      "clearConsoleOnReload": true,
      "reAttach": true,
      "reloadOnAttach": true,
      "keepProfileChanges": false,
      "reloadOnChange": "${workspaceFolder}/src/js/**",
      "url": "https://www.google.com/",
      "addonPath": "${workspaceFolder}\\tests\\firefox",
      "pathMappings": [
        {
          "url": "webpack:///js",
          "path": "${workspaceFolder}/src/js"
        }
      ]
    }
  ]
}