If you are developing JavaScript, it might interest you to know that there is a good JavaScript debugger available for the Firefox browser. To install it follow these steps:

1. Open Firefox
2. In the menu, click on Tools/Extensions
3. In the window, click on 'Get more extensions'
4. Paste this url into the address line: https://addons.mozilla.org/firefox/216/
5. Click on 'Install Now'
6. Follow the instructions

To start the debugger:
1. Open the page that contains your JavaScript
2. In the menu, click on Tools/JavaScript debugger

There are several other useful tool available, if you build html from your JavaScript, I can recommend the DOM Inspector.