Did you download this from firefox.com or did you get this from Help -> Updates? The Updates option should not require you to copy anything, it should just install. If you downloaded it, do you have FF running when you try to copy it over? (I did that once with the same error.) If both of those fail, can you delete FF in the Applications folder? (try doing this after a restart so you know all programs are closed) If so, do that then drag the new one into place. You'll also probably have to replace your dock icon if you have one. Drag the old one off, then drag the new one on. Also, are you trying to drag in the dmg file or are you mounting the dmg? With a dmg, you should double click to mount it as a drive first. It should then open the drive image for viewing and give you the option to drag the FF icon to the Applications symlink. (shortcut) You don't drag it out of the window into the Applications window, you just drag it over the Applications shortcut icon and it makes the move for you. Failing all of that, try re-downloading the app from firefox.com. Perhaps you got a bad download with errors. If that doesn't work as well, then you can resort to using sudo cp on the command line in a terminal. If it gets that bad, post that back here and someone or myself can walk you through it if you aren't familiar with doing that. It isn't as scary as it sounds, but it is best to try to use the normal methods first.