If you set PDF to always ask it won't open in the internal viewer but it will ask to download, why not setting it to Open in Nightly? Then you can use the download button on the viewer to download if you wish to keep it. As you tried to setup, it will end up in a dialog loop where firefox asks, you tell to open with firefox, firefox asks, and so on... that loop is cut short just downloading then. If you don't want to keep the files by default, there is a browser.download.start_downloads_in_tmp_dir option in about:config, though we think the default behavior is less likely to cause issues and dataloss. Does this help?
Bug 1790506 Comment 2 Edit History
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If you set PDF to always ask it won't open in the internal viewer but it will ask to download, why not setting it to Open in Nightly? Then you can use the download button on the viewer to download if you wish to keep it. As you tried to setup, it will end up in a dialog loop where firefox asks, you tell to open with firefox, firefox asks, and so on... that loop is cut short just downloading to avoid the problem. If you don't want to keep the files by default, there is a browser.download.start_downloads_in_tmp_dir option in about:config, though we think the default behavior is less likely to cause issues and dataloss. Does this help?