hext.html game fails, "window.event" error in Firefox

In Firefox 23.0 running http://sharejs.org/hex.html#dPtvIpWA=g , it doesn't work and I get

```
TypeError: e is undefined
http://sharejs.org/hex.html
Line 369
```

The line is

```
if (!e) var e = window.event;
if (e.pageX || e.pageY) {
```

There's no window.event in Firebug. There is a `window.Event` (capitalized) but it doesn't have pageX member; meanwhile the `ev` passed in does.