core-item ignores clicks on polymer-project.org in Firefox

[nhnb]

Steps to reproduce:

1. Go to https://www.polymer-project.org/docs/elements/ in Firefox 33.0 on Ubuntu
2. Click on "Elements" in the left hand navigation menu

Expected result

The Elements section should open and present submenus for core-elements and paper-elements.

Actual result

The click event is ignored. Nothing changes.

Related information

It works on both Chrome desktop and Chrome mobile and on Firefox mobile. It used to work a couple of days ago on Firefox desktop, too.

[frankiefu]

duplicate of #941