Backout Bug 1321865 because of stability issues, r=backout

modules/libpref/init/all.js

@@ -5558,4 +5558,4 @@ pref("prompts.authentication_dialog_abuse_limit", 3);
 // To enable the DOM implementation, turn on "dom.storageManager.enabled"
 pref("browser.storageManager.enabled", false);
 
-pref("dom.IntersectionObserver.enabled", true);
+pref("dom.IntersectionObserver.enabled", false);