Getting the current page URL can be dependend on the server set up. The standard function (which may not always work! - read on if your stuck!) goes like this:

This normally occurs with drop down menus being hidden behind other elements on the page.

There are two issues to watch out for. Firstly, z-index WILL solve it. But z-index only works on elements that are positioned (this is required for firefox). So you may need to insert 'postion: relative;' into both the Menu style and the object that it gets hidden behind.