7-zip is used to compress files in the installer. If it says it can't find or access the files, it could indicate that after the files were decompressed, your antivirus quarantine the files, or Windows locked them for some reason. ??

Do you want to try the full installer from this page:

https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Yes, try the full setup as some antivirus and such scanners can have issues with the small online stub installer.

jscher reply gave me the clue. I was installing Firefox 14 update, as usual, from a non Administrative account as have been doing for many years. This was failing because the update no longer brings up the Admin password request to allow changes to the PC as it has been doing in the past thus 7-zip (unzip) was not able to write files to the folder for later use. I logged in as administrator and it installed the update without problem. Perhaps Mozilla can look into this and fix it. Many thnaks for everyones feedback. SG

Hi SG, you shouldn't need to install Firefox as the administrator (or as "an" administrator), but I guess once you have, then you're stuck with always doing it that way???