My Firefox was not working, so I deleted it and planned to reinstall it. I downloaded Firefox using Safari. When I try to reinstall, I get the above message.
Try to repair the disk permissions:
If there are problems with updating or with the permissions then easiest is to download the full version and trash the currently installed version to do a clean install of the new version.
Download a new copy of the Firefox program and save the disk image (dmg) file to the desktop
Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data if you uninstall and (re)install Firefox.