abrowser: disable firefox marketplace

Author: g4jc
Source: https://trisquel.info/en/issues/13405

helpers/DATA/firefox/settings.js

@@ -169,3 +169,5 @@ pref("middlemouse.contentLoadURL", false);
 //Disable heartbeat
 pref("browser.selfsupport.url", "");
 
+//Disable Link to FireFox Marketplace, currently loaded with non-free "apps"
+pref("browser.apps.URL", "");