Enables the screen capture on firefox.

modules/RTC/RTCUtils.js

                     }
                 ]
             };
+        } else if (RTCBrowserType.isFirefox()) {
+            constraints.video = {
+                mozMediaSource: "window",
+                mediaSource: "window"
+            };
+
         } else {
             console.error(
                 "'screen' WebRTC media source is supported only in Chrome" +

modules/desktopsharing/desktopsharing.js

             obtainDesktopStream = obtainWebRTCScreen;
             console.info("Using Chrome WebRTC for desktop sharing");
         }
+    } else if (RTCBrowserType.isFirefox()) {
+        obtainDesktopStream = obtainWebRTCScreen;
     }
 
     if (!obtainDesktopStream) {