--- a/layout/tools/reftest/reftest-preferences.js
+++ b/layout/tools/reftest/reftest-preferences.js
@@ -132,18 +132,16 @@ user_pref("testing.supports.moz-bool-pre
 // unnecessary I/O pressure on the Places DB (measured to be in the
 // gigabytes).
 user_pref("places.history.enabled", false);
 
 // For Firefox 52 only, ESR will support non-Flash plugins while release will
 // not, so we keep testing the non-Flash pathways
 user_pref("plugin.load_flash_only", false);
 
-user_pref("media.openUnsupportedTypeWithExternalApp", false);
-
 // Disable all recommended Marionette preferences for Gecko tests.
 // The prefs recommended by Marionette are typically geared towards
 // consumer automation; not vendor testing.
 user_pref("marionette.prefs.recommended", false);
 
 // Make sure we don't reach out to the network with pocket or snippets
 user_pref("browser.newtabpage.activity-stream.feeds.section.topstories", false);
 user_pref("browser.newtabpage.activity-stream.feeds.snippets", false);

--- a/mobile/android/app/mobile.js
+++ b/mobile/android/app/mobile.js
@@ -912,18 +912,16 @@ pref("dom.presentation.receiver.enabled"
 pref("dom.audiochannel.audioCompeting", true);
 pref("dom.audiochannel.mediaControl", true);
 pref("media.block-autoplay-until-in-foreground", false);
 
 // Space separated list of URLS that are allowed to send objects (instead of
 // only strings) through webchannels. This list is duplicated in browser/app/profile/firefox.js
 pref("webchannel.allowObject.urlWhitelist", "https://accounts.firefox.com https://content.cdn.mozilla.net https://input.mozilla.org https://support.mozilla.org https://install.mozilla.org");
 
-pref("media.openUnsupportedTypeWithExternalApp", true);
-
 pref("dom.keyboardevent.dispatch_during_composition", true);
 
 #if CPU_ARCH == aarch64
 pref("javascript.options.native_regexp", false);
 #endif
 
 // Ask for permission when enumerating WebRTC devices.
 pref("media.navigator.permission.device", true);

--- a/testing/profiles/prefs_general.js
+++ b/testing/profiles/prefs_general.js
@@ -368,18 +368,16 @@ user_pref("startup.homepage_welcome_url.
 // For Firefox 52 only, ESR will support non-Flash plugins while release will
 // not, so we keep testing the non-Flash pathways
 user_pref("plugin.load_flash_only", false);
 
 // Don't block old libavcodec libraries when testing, because our test systems
 // cannot easily be upgraded.
 user_pref("media.libavcodec.allow-obsolete", true);
 
-user_pref("media.openUnsupportedTypeWithExternalApp", false);
-
 // Disable password capture, so that mochitests that include forms aren't
 // influenced by the presence of the persistent doorhanger notification.
 user_pref("signon.rememberSignons", false);
 
 // Enable form autofill feature testing.
 user_pref("extensions.formautofill.available", "on");
 
 // Disable all recommended Marionette preferences for Gecko tests.