After viewing some few email attachments in overlay, page content freezes. Sometimes Gmail overlay is semi-transparent and must be closed and reopened. I am not visiting many pages with overlays so don't know if this is specific for Gmail only. Tried refreshing FF, creating new profile and working without any addon until problem occurred again. This started more than month ago. It seems the overlay is not dismissed properly. Opening developer tools or resizing window makes the main page content available again.