F/fox now comes with something they call Live bookmarks.
Under Bookmarks | Bookmarks Toolbar Folder |Latest headlines you will see links to latest updated news headlines.
When you write ZA 'opening' do you mean it is showing some data downloading?
F/fox could be updating these headlines.
Dont know if IE has this feature. (Too scared to open it these days.) Maybe this is making the diff in behaviour of ZA?