--- a/testing/web-platform/meta/mediacapture-streams/GUM-deny.https.html.ini
+++ b/testing/web-platform/meta/mediacapture-streams/GUM-deny.https.html.ini
@@ -1,4 +1,5 @@
 [GUM-deny.https.html]
+  prefs: [media.getusermedia.camera.deny:true]
   [Tests that the error callback is triggered when permission is denied]
     expected: FAIL
 

new file mode 100644
--- /dev/null
+++ b/testing/web-platform/meta/mediacapture-streams/__dir__.ini
@@ -0,0 +1,2 @@
+prefs: [media.navigator.permission.disabled:true,
+        media.navigator.streams.fake:true]