Merge pull request #1573 from saghul/enable-ff-ds

fix(screen-sharing) Enable it by default on Firefox

config.js

     // extension is required.
     desktopSharingFirefoxExtId: null,
     // Whether desktop sharing should be disabled on Firefox.
-    desktopSharingFirefoxDisabled: true,
+    desktopSharingFirefoxDisabled: false,
     // The maximum version of Firefox which requires a jidesha extension.
     // Example: if set to 41, we will require the extension for Firefox versions
     // up to and including 41. On Firefox 42 and higher, we will run without the