Or maybe, bwear, the "stylesheet_categories_menu.css" got corrupted during FTP, as it's not loading in FF or IE. And if it's not loading, well.............then it would not look OR act right. Check it, make sure all the information's there, maybe re-upload it to your server? Also might double-check the rest of the files for the menu, as well, making sure they're in the correct places with all the correct info?

Did you change anything, yet (on the menu), or is it "out of the box", still?