Top-level menu items not clickable in Firefox

I'm trying to set up NiceMenus on a test site, and have run into an odd problem - in Firefox, the top-level menu items can't be clicked on, or at least, not for more than a half-second or so.

When you first mouse over them, the mouse pointer changes the way it usually does to indicate a link - but only for half a second, and then it switches back to a regular pointer and doesn't seem to acknowledge the top item as a link in any way (i.e., no destination shows up in the status bar and nothing happens if you try to click). The submenus under them work normally, and so do any top-level items that don't have submenus. But any top-level item that has a submenu does not appear as a link.

The problem appears to be specific to Firefox, but happens across different platforms (i.e. Firefox for Mac, Windows and Linux all show the same behaviour, but other browsers do not). It also happens regardless of whether Javascript is enabled or not - so it's not specifically a Javascript issue, it also happens if it's displaying as a pure CSS menu.

Has anyone else encountered this? I assume it can't be happening to everyone, or I would have been able to find other posts about it in the issues queue... The test site where I'm seeing it is http://opc-ref.spidersilk.net