I would say this is a Windows issue, and like most other programs, Firefox needs access to that folder, even when the actual profile data is located elsewhere. In other words, if a virus or other piece of software is preventing access to the %AppData% folder, Firefox will not be the only application the issue occurs for. You can try the following in this order: - First, run a virus scan and make sure the technician has a point. ;) - Reboot your system and try updating Firefox again. - Untick "Use a background service to install updates" in The Firefox Options page > General panel > Firefox Updates section and retry. You should also be able to install Firefox on top of the current install after downloading it from https://www.mozilla.org/firefox/all/. Please report back if you need further assistance.