--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -2112,17 +2112,26 @@ nsPresContext::MediaFeatureValuesChanged
   // flushes.  (We're already running off an event.)
   //
   // Note that we do this after the new style from media queries in
   // style sheets has been computed.
 
   if (!mDocument->MediaQueryLists().isEmpty()) {
     // We build a list of all the notifications we're going to send
     // before we send any of them.
-    for (auto mql : mDocument->MediaQueryLists()) {
+
+    // Copy pointers to all the lists into a new array, in case one of our
+    // notifications modifies the list.
+    nsTArray<RefPtr<mozilla::dom::MediaQueryList>> localMediaQueryLists;
+    for (auto* mql : mDocument->MediaQueryLists()) {
+      localMediaQueryLists.AppendElement(mql);
+    }
+
+    // Now iterate our local array of the lists.
+    for (const auto& mql : localMediaQueryLists) {
       nsAutoMicroTask mt;
       mql->MaybeNotify();
     }
   }
 }
 
 void
 nsPresContext::PostMediaFeatureValuesChangedEvent()