Oh Logress was having the same issue, you have to close out of the old pop-up. If you have a window, say 600 pixels wide by 380 pixels high and you try to view another card's details, it will fit in the old window and not make a new one. Close out all instances of Firefox, reload and try it again. You may have to refresh the browser but it should work fine. We use Firefox 3 over here as well and it fits fine. 380x600. We flipped the dimensions.
P.S. I thank Chronomaster for changing his avatar. That thing creeped me out.
"what on earth did you just say? did someone nerf your keyboard?" ~Spears
I feel that way all the time Spears, all the time.