hi,
for all those who searches up thier asses to solve the problem, the submenu, on hover, shown in internet explorer older 9,
you have to put the div, containing the megamenu, to a higher z-index, as the rest of the content!
In my case it was the header which hasn't any z-index, but the content has z-index: 1. In firefox this doesn't matter.
Since i have put the header div, containning the megamenu, to z-index: 2. the megamenu also is shown in internetexplorer lower 9.