message: add run:firefox in npm scripts
diff: "release": "npm run build:chrome:prod && cross-env NODE_ENV=prod npm run pack:chrome && npm run build:firefox:prod && npm run pack:firefox",
  "release:firefox": "npm run build:firefox:prod && npm run pack:firefox && rimraf artifacts/dist-firefox.zip",
  "release:chrome": "npm run build:chrome:prod && cross-env NODE_ENV=prod npm run pack:chrome",
  + "run:firefox": "web-ext run -s dist",
  "lint": "eslint . && stylelint src/**/*.css",
  "test": "npm run lint && npm run release",
  "update_emojis": "node tools/update_emojis.js",
  