Windows should remove the folder’s content by default, I think once about every few months or so, given the dates of most recent files I can spot on a similar OS. Additionally, this folder’s content can always be cleared manually at any time, or by some batch file run frequently or at startup, or even automatically by some tools such as CCleaner, probably. It’s pure a caching folder, and any files removed should be recreated when needed. The xpi files apply to installs or updates from add-ons - you should even be able to view their content as zip files. Also see [https://superuser.com/questions/571901/is-it-safe-to-delete-appdata-local-temp/902645 this question @ superuser<!-- -->.com].