Could you please attach to this report a copy of the handlers.json file from the Firefox profile in question? It would also be useful to get either a screencast, or alternatively a screenshot of the "Open/Save" dialog or panel, in case you see one. Do you see a handler for RSS in the Applications section of about:preferences and how is it set (again a screenshot would help)?
Bug 1844463 Comment 2 Edit History
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Could you please attach to this report a copy of the handlers.json file from the Firefox profile in question (can retrieve the profile folder from about:support)? It would also be useful to get either a screencast, or alternatively a screenshot of the "Open/Save" dialog or panel, in case you see one. Do you see a handler for RSS in the Applications section of about:preferences and how is it set (again a screenshot would help)?