message: add firefox build script
diff: "scripts": {
  "start": "webpack-dev-server --hot",
  "build": "webpack",
  - "build-chrome": "rm -rf build && env NODE_ENV=production yarn build && ./build.sh build ./selenium-ide.pem && mkdir -p dist && mv build.crx dist/selenium-ide.crx",
  + "build-prod": "rm -rf build && env NODE_ENV=production yarn build",
  + "build-chrome": "./build.sh build ./selenium-ide.pem 1>/dev/null && mkdir -p dist && mv build.crx dist/selenium-ide.crx && echo \"Wrote dist/selenium-ide.crx\"",
  + "build-firefox": "web-ext build -s build -a dist",
  "test": "jest",
  "test-dev": "jest --watch",
  "lint": "eslint src/neo/ --ext .js --ext .jsx"
  