The simplest way to get started with Firefox OS is to try out the App Manager. This one is part of the Developer Tools of Firefox for Desktop and simulates a Firefox OS device on your computer. You can play with Firefox OS and install and test applications just like you would on a real device. Furthermore, you can try out and debug your own applications by editing them offline and online on your computer and see the changes live inside the simulated device without any hassle.
More reading:
Open Web Apps App Manager: https://developer.mozilla.org/en-US/Firefox_OS/Using_the_App_Manager
Firefox Developer Tools: https://developer.mozilla.org/en-US/docs/Tools/
