I recently upgraded someone's computer to a later version of windows 10. They had Firefox 102.6esr and I upgraded that to 102.15.1. They didn't want to upgrade to 115 yet for a number reasons. This was an in-place upgrade. After the upgrade all attempts to install an addon (Let's take canvasclocker as an example), or any theme (I tried many) all throw an error in a red bar saying an unexpected error occurred. I followed this: https://support.mozilla.org/en-US/kb/unable-install-add-ons-extensions-or-themes And method B from here: https://www.askvg.com/how-to-fix-unexpected-installation-error-203-problem-while-installing-extensions-and-themes-in-mozilla-firefox/ I also saw a post somewhere I can't recall about downloads being blocked so I checked about:config but didn't see anything unusual (compared to my settings). What might be causing the error?