Error message: "Update could not be installed. Please make sure no other copies of Firefox running on your computer and restart Firefox again."
There are no other copies running. At least none that I know of - how do I check - and if not, what else would be the problem/how can I fix?
Please go to www.getfirefox.com, download Firefox 19.0. Then, restart your Pc, then install the Firefox update.
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.