Site gets stuck loading

Hi, I’ve just installed this new theme and every once in a while going through different posts in the theme the site locks up in FireFox. The loading circle is still moving like its hung up trying to load something. Once I hit stop, the site loads everything up like the menu and sharebar, etc. Why does this happen?

You can try to emulate this in FireFox: http://robertsproductions.net
Click through a bunch of posts and pages. It’ll probably happen.

I’ve actually added a custom ad in the header, seems like this doesn’t happen when I take the ad out. No matter where I place the ad in the header.php file, it still does this.