This behaviour comes from a change made in Bug 1710933: https://searchfox.org/mozilla-central/rev/bc5e79f3ae0f42cb4a6ebd05fc32f48a3829059d/toolkit/components/downloads/DownloadCore.jsm#607-615. Since we're not downloading externally handled files to the `tmp/` directory anymore, we don't delete these files when the browser session is closed. However, we may still honor "browser.helperApps.deleteTempFileOnExit" if we also check if the file is a temporary download in `Download._succeed`. A similar check is done [here](https://searchfox.org/mozilla-central/rev/bc5e79f3ae0f42cb4a6ebd05fc32f48a3829059d/toolkit/components/downloads/DownloadIntegration.jsm#638-647).
Bug 1741929 Comment 1 Edit History
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This behaviour comes from a change made in Bug 1710933: https://searchfox.org/mozilla-central/rev/bc5e79f3ae0f42cb4a6ebd05fc32f48a3829059d/toolkit/components/downloads/DownloadCore.jsm#607-615. Since we're not downloading externally handled files to the `tmp/` directory anymore, we don't delete these files when the browser session is closed. We may still honor "browser.helperApps.deleteTempFileOnExit" if we also check if the file is a temporary download in `Download._succeed`. A similar check is done [here](https://searchfox.org/mozilla-central/rev/bc5e79f3ae0f42cb4a6ebd05fc32f48a3829059d/toolkit/components/downloads/DownloadIntegration.jsm#638-647). But again, since we're not downloading to the `tmp/` directory, it doesn't make sense that we should do be doing this check just because it's set to true. We can remove this preference to avoid confusion and perhaps we can expose data on a download that indicates it was opened automatically (see Gijs' comment in https://bugzilla.mozilla.org/show_bug.cgi?id=1738574#c3).