At some point, Firefox changed the old behavior of downloading opened files temporarily only, and now all files are saved and kept by default. This pollutes the user home directory with files that the user might not have wanted to keep. Even keeping the current behavior as default, having the option visible in preferences will help users understand the browser behavior. It has received sufficient backlash from the community to be considered.
Since it is already in about:config, not much effort would go into officially supporting it, but it would guarantee that the option does not quietly disappear. Now, users unsatisfied with this behavior need to search in the internet for a solution that is not guaranteed to exist in futures releases, when it could be another checkbox in the Files and Applications section in Preferences.
Examples of the community take and confusion over this matter:
https://unix.stackexchange.com/questions/705045/open-files-without-download-them-in-firefox
https://stackoverflow.com/questions/78871696/open-in-firefox-should-save-file-in-temporary-folder-but-saves-it-in-my-downlo
https://www.askvg.com/tip-force-firefox-to-start-downloads-in-temp-folder-again/
https://www.reddit.com/r/firefox/comments/1941hi2/firefox_downloading_files_to_default_download/
https://connect.mozilla.org/t5/ideas/unhide-browser-download-start-downloads-in-tmp-dir-and-add-to/idi-p/45899
https://www.reddit.com/r/firefox/comments/ww0ms1/why_is_firefox_both_opening_and_downloading/
https://connect.mozilla.org/t5/ideas/unhide-browser-download-start-downloads-in-tmp-dir-and-add-to/idi-p/45899
and many more.
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