Up until Firefox version 97, files opened but not downloaded were saved on a hidden "temp" folder, deleted automatically after restart. Since Firefox version 98, they are all downloaded to a visible folder, either chosen by the user or the "Download" one, in Windows/Linux. A super annoying change. In their own words:
How can I manually revert this change? Settings have no option related to this.
browser.download.improvements_to_download_panel
: support.mozilla.org/en-US/questions/1370409