What most probably happened then was that you downloaded firefox mounted the disk image opened firefox from the disk image then clicked keep in dock, then correct me if I'm wrong installed it properly by dragging it to the applications folder. Then re clicked the dock icon to launch firefox correct? What happened was by re clicking the dock icon you where clicking the shortcut for the file (firefox) that was located on the disk image. Which meant the disk image had to be remounted to open the file (firefox), what happened was when you first launched firefox you where launching the firefox on the disk image and you clicked keep in dock so you where keeping the disk image file (firefox) in the dock, but when you installed it properly by dragging it to the apps folder you made a copy of it which was now located in the applications folder so when you clicked the icon on the dock it open the firefox on the disk image because that is what the shortcut was for. If you had removed it from the dock launched it from the applications folder and clicked keep that in dock it would have solved the problem. Do you see what I'm saying here?
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