We're seeing reports from confused users who have their PDF settings at "always ask" and do not recall changing them to that setting. Bug #1660277 has some concrete steps that cause this to happen. but those steps regressed recently. Given the reports came from release/esr users, I suspect there are other steps that cause the same problem. This bug is to track that and figure out what other steps (if any) cause this to happen...

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:Gijs, could you have a look please?

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So bug 1660277 dealt with the concrete STR we're aware of here. I don't see a possibility for other accidental changes here, unless the user ticks the "always do this" checkbox, in which case those changes should happen.

We only store things from the dialog when updateHelperAppPref gets called, which only happens at https://searchfox.org/mozilla-central/rev/0c97a6410ff018c22e65a0cbe4e5f2ca4581b22e/toolkit/mozapps/downloads/HelperAppDlg.jsm#1093-1098 , which only happens if updateMIMEInfo returns true (at https://searchfox.org/mozilla-central/rev/0c97a6410ff018c22e65a0cbe4e5f2ca4581b22e/toolkit/mozapps/downloads/HelperAppDlg.jsm#1073 ) which only happens if discardUpdate is false, and that (now) only happens if either the checkbox is set or we're not set to handle PDFs internally.