Firefox Private Browsing & Window Raising

I've noticed several quirks when using Firefox. This is one of the more recent and annoying ones.

I first noticed it on windows, when there is more than one window open. In my case this means one "normal" window and one private browsing window. What will happen is that I'll load a particular site and at some point during the process of the page load the other window will get raised to the foreground. It is as if there's a "window.focus()" javascript call in the page that I'm loading but it's raising the other window instead of the active window. So far it's been impossible to figure out what precisely is happening so I can stop it. Instead I periodically have to minimize my other Firefox window.