VERY Irritating PROBLEM! When I minimize browser pages then add a new page all of the trayed pages pop up. They cascade and cover up the screen. I uninstalled Firefox and re installed latest download and it still does it.
I have been having this same problem for ages BUT I think I have solved it. A while back I was trying to save memory and one of the settings that I tried, and forgot about, was config.trim_on_minimize in about:config . I had it set to "true" but now that I have set it back to the default of "false" the problem seems to be, for me anyway, fixed.