To avoid this error, you must be updating Firefox from the account you installed it on. If you update it from an administrator account other than the one you originally used to install it, you will get this message.

To update with this user, you simply need to uninstall and re-install Firefox. Automatic updates will then work properly. Specific instructions are at Uninstall Firefox from your computer and How to download and install Firefox on Mac

I AM updating from the account I installed Firefox on-- and I'm still having the identical problem as Phil. This is clearly a bug on Firefox's side with the latest edition-- I've updated previous versions on the same computer/same user before.

I don't want to go through the aggravation of uninstalling, reinstalling, and trying to recover lost bookmarks etc...

You won't lose your user profile that stores your bookmarks and other personal data. You only need to trash the current Firefox installation and install the new version. --- See http://kb.mozillazine.org/Installing_Firefox#Mac_OS_X ..... Download a new copy of the Firefox program: http://www.mozilla.com/firefox/all.html Trash the current Firefox application to do a clean reinstall. Install the new version that you have downloaded. Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data.