Issue in Firefox 22

I just figured out there's a small problem with v2.6 in Firefox 22. Window height from jQuery and document height return different values in Firefox, causing the popup to resize improperly, while they return absolutely the same value in Chrome. I attached two images to show the problem. The first image shown is the correct render in Chrome browser, v28, and the second one is the improper render from Firefox v22.

In this branch I made a small fix for this.

![chrome_lightbox2_correct](https://f.cloud.github.com/assets/698465/795489/05286e10-ecfb-11e2-961a-e71523eb7081.jpg)
![firefox_lightbox_issue](https://f.cloud.github.com/assets/698465/795491/0a2a3254-ecfb-11e2-82fc-c91eb6060e62.jpg)

Thanks for the attention!