This question already has an answer here:
When I got on today, the top left header menus were gone. The places they should be seem to still be functional, but it looks like just a black bar. Browser is Firefox 27, OS is Windows 7.
Here is a screenshot of main SO
Just so we are clear, this is only the 3 menus in the top left. The user image and badges are still fine.
When new activity happens, it puts up the red background with the number in it.