QA/Performance fixes

Summary

• Mozilla is constantly searching ways to improve the user experience of browsing the web. Better response time, loading pages faster, turn on/off Firefox faster, switching between tabs/menus smoother. All this goals under a sugestive name "Performance Fixes".

Reference

• • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=718088">bug 718088</a> - (asyncContentPrefs) Refactor nsContentPrefService.js to use async Storage
  • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=818296">bug 818296</a> - [Shutdown] js::NukeCrossCompartmentWrappers takes up to 300ms on shutdown. Avoid doing it for optimized shutdown
  • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=818739">bug 818739</a> - Don't run CC during shutdown
  • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=740468">bug 740468</a> - Replace old synchronous favicons calls in the bookmarks export service
  • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=789932">bug 789932</a> - nsExternalAppHandler downloads files on the main thread
  • <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=807021">bug 807021</a> - Move LocalStorage writes off the main thread

Sign off Criteria

TBD