Just go this page, and install the ADB Helper by clicking Install ADB Helper. After that open WebIDE from Tools->Web Developer->WebIDE. Click Select Runtime from top right. Under the SIMULATORS section, you’ll see Install Simulator option, just click that. In this window, you can see available simulators, install the version you want.
After installing the simulator, click Select Runtime again, you’ll see your device under the SIMULATORS section. Click and open it. While it’s opening, you can get an error but the simulator will run. (There is a bug record about this)
Now you are ready to write your first app. Firefox WebIDE offers all the thing you need. To create new app, click Open App from top left and select New App. This will show you some templates, for now, select HelloWorld template and set your project path.
WebIDE will open your project, you can see your project tree on the left side. The project is pretty simple, just writes Hello World to screen. To run your project, just click play button and it will show up in simulator(Don’t forget to save your files before running your app, just do ctrl + s before running).