After a recent update, hovercards pop up above the mouse cursor if the hovered link is located on the bottom half of the page.
The problem is, that the hovercard (or the small tip to be exact, see screenshot) is generated directly under the mouse cursor.
This results in fast flickering of the hovercards since as soon as the hovercard pops up, the link is shadowed, therefore triggering the hovercard to be hidden. As soon as the hovercard is hidden, the link is not shadowed anymore, triggering the hivercard to be shown again
The above happens for me on Windows 7 using Firefox 29.0.1
(I tried the same with Internet Explorer 11 but there the hovercards where pretty much broken anyway)