Weird pop up authentication when I open 1 thread

[.invisible ink.]

This is really creepy. I went into a thread this morning in General called "Elaborate Plan to Get Rid of Annoying Date" and the following authentication box came up. I thought it was weird, closed my browser, deleted the "sandbox" (I use Sandboxie) that Firefox is housed in, and came back and had the same exact thing happen again.

Any explanation that you could think of or has my computer been compromised in some way?

http://i123.photobucket.com/albums/o306/RollrXGirl/weirdpopuponap.jpg