So How to Run Firefox OS in Firefox browser?
1) Open your firefox browser.
2) Open this link. It will get you to the add-on page.
3) Click add to firefox.
4) The add-on will be downloaded and ask your authorization to be installed. Click install.
5) The Simulator dashboard will be opened automatically. Switch it to Running by clicking the switch.
That’s all the Firefox OS simulator will be opened. You can try Firefox OS.