I just powered up my first Mac earlier this morning. I've downloaded and installed (I think) Firefox and Thunderbird. I followed the on screen instructions and drug each app's icon to the Application Folder. I then drug each icon onto the Dock. Each of the applications ran when I clicked their icon, so I felt safe in deleting the .dmg files. I drug each of the .dmg files to the trash, but when I tried to empty the trash it informed me that both of these files were in use. What am I missing?