In bug 1639067 I enabled the option to open a downloaded file immediately in Firefox after the download finished, which was previously possible for PDF with the browser.helperApps.showOpenOptionForPdfJS pref. This would replace the previous default, which was to save the file. This was done to keep things more consistent with PDF, and to show what the preference was for the MIME type.

But since this becomes the default, it may be surprising to many users, and especially given that this interface is usually shown when offered the option to "save" a file from a web site (usually resulting in content-disposition: attachment), this surprise may lead to data loss as the file is only saved temporarily.

Bug 1632274 proposes to solve this by not automatically deleting the temporary file (or by not saving it in a temporary location), but until the UI issues for that are resolved I think it would be best to not show this option, or to not select it by default. If the user doesn't select "Do this automatically..." then the underlying setting to open internally (which takes effect when opening the file from the downloads list) will still be present, consistent with what happens with PDF.

The best option for now is probably a new pref, browser.helperApps.showOpenOptionForViewableInternally, false by default.

I talked through this with Gijs and we agreed to not disable this yet. The patch still has the pref, but defaulted to true. Any redesign of this would likely involve PDF as well, and so would be included in bug 1632274.

https://hg.mozilla.org/mozilla-central/rev/3b40d577652f