Hi Khaledfawzy, This feature is not natively available on Firefox OS simulator. I was able to find a couple of links on StackOverflow and Github that may give you some workarounds and suggestions: * [http://stackoverflow.com/questions/17097814/how-to-handle-different-screen-resolutions-in-firefox-os StackOverflow - How to handle different screen resolutions in Firefox OS] * [https://github.com/mozilla/r2d2b2g/issues/272 Github - Cannot set screen resolution of the simulator] You can also follow the development of this feature in the following bugzilla tiket: * [https://bugzilla.mozilla.org/show_bug.cgi?id=948056 Bug 948056] I hope you find this information useful. Please let us know if you have any usage questions about the Firefox OS system. Thanks, - Ralph

Hi Khaledfawzy, Please see the following instructions on MDN: * [https://developer.mozilla.org/en-US/Firefox_OS/Building_the_B2G_desktop_client#option-screen Building the B2G desktop client - option screen] - Ralph