I have two windows open, but one will only show the preview and when I try to bring it up, it won't open?

I can see the window as a little thumbnail when I hover over the firefox icon in the taskbar - & I can even change the contents, because it seems to be the 'main' window, so when I tried to change a setting in my add-ons, the add-on window opened in it, and I have no way of reaching it. When I try to click on the window to bring it up, it 'selects' and is highlighted, but it never comes up, no matter how long I wait. I can close the window that does pop up, and the other window remains unaffected. Even manually trying to "switch over" to the window from the task manager won't work.

EDIT: I am all up-to-date, Firefox 15.0, etc.