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Firefox doesn't display the "Stack Exchange Sites", "Recent Inbox Messages" and "Recent Achievements" popup buttons located in the menu bar at the top. The buttons are there - just invisible, though. Hovering over with the mouse will display the tool tips, and they work as expected.
This is Firefox 28 on Windows 7 SP1 (usually, I'm on Safari, and there is no issue).
How do I solve this issue?