I am now using Firefox but am having the above problem updating it.
Sorry to read that you are having difficulties to download Firefox.
Are you using the link in Mozilla.org?
Is there any pop up starting the download? The message that you are posting is unfamiliar to me.
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.