From my point of view, the setting " What should Firefox do with other files?" has been added in the current ESR version. "What should Firefox do with other files?" ("Wie soll Firefox mit anderen Dateien verfahren?") . "Save files" ("Dateien speichern") . "Ask whether to open or save files" ("Fragen, ob Dateien geöffnet oder gespeichert werden sollen") How can I control/change this setting using mozilla.cfg? By the way: // What should Firefox do with other files? - Wie soll Firefox mit anderen Dateien verfahren? lockPref("applications-ask-before-handling", false); // What should Firefox do with other files? - Wie soll Firefox mit anderen Dateien verfahren? lockPref("applications-ask-before-handling", true); works detectably via about:config but does not change the setting for "What should Firefox do with other files?".