diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 849081693ded..8a192e97c2df 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -10429,6 +10429,11 @@ nsresult nsDocShell::OpenInitializedChannel(nsIChannel* aChannel,
   rv = aURILoader->OpenURI(aChannel, aOpenFlags, this);
   NS_ENSURE_SUCCESS(rv, rv);
 
+  // We're about to load a new page and it may take time before necko
+  // gives back any data, so main thread might have a chance to process a
+  // collector slice
+  nsJSContext::MaybeRunNextCollectorSlice(this, JS::GCReason::DOCSHELL);
+
   // Success.  Keep the initial ClientSource if it exists.
   cleanupInitialClient.release();
 
diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 7ab520b6eaf2..20c1093d38a3 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1738,6 +1738,64 @@ void nsJSContext::RunNextCollectorTimer(JS::GCReason aReason,
   }
 }
 
+// static
+void nsJSContext::MaybeRunNextCollectorSlice(nsIDocShell* aDocShell,
+                                             JS::GCReason aReason) {
+  if (!aDocShell || !XRE_IsContentProcess()) {
+    return;
+  }
+
+  BrowsingContext* bc = aDocShell->GetBrowsingContext();
+  if (!bc) {
+    return;
+  }
+
+  BrowsingContext* root = bc->Top();
+  if (bc == root) {
+    // We don't want to run collectors when loading the top level page.
+    return;
+  }
+
+  nsIDocShell* rootDocShell = root->GetDocShell();
+  if (!rootDocShell) {
+    return;
+  }
+
+  Document* rootDocument = rootDocShell->GetDocument();
+  if (!rootDocument ||
+      rootDocument->GetReadyStateEnum() != Document::READYSTATE_COMPLETE ||
+      rootDocument->IsInBackgroundWindow()) {
+    return;
+  }
+
+  PresShell* presShell = rootDocument->GetPresShell();
+  if (!presShell) {
+    return;
+  }
+
+  nsViewManager* vm = presShell->GetViewManager();
+  if (!vm) {
+    return;
+  }
+
+  // GetLastUserEventTime returns microseconds.
+  uint32_t lastEventTime = 0;
+  vm->GetLastUserEventTime(lastEventTime);
+  uint32_t currentTime = PR_IntervalToMicroseconds(PR_IntervalNow());
+  // Only try to trigger collectors more often if user hasn't interacted with
+  // the page for awhile.
+  if ((currentTime - lastEventTime) >
+      (StaticPrefs::dom_events_user_interaction_interval() *
+       PR_USEC_PER_MSEC)) {
+    Maybe<TimeStamp> next = nsRefreshDriver::GetNextTickHint();
+    // Try to not delay the next RefreshDriver tick, so give a reasonable
+    // deadline for collectors.
+    if (next.isSome()) {
+      nsJSContext::RunNextCollectorTimer(aReason, next.value());
+    }
+  }
+}
+
 // static
 void nsJSContext::PokeGC(JS::GCReason aReason, JSObject* aObj,
                          uint32_t aDelay) {
diff --git a/dom/base/nsJSEnvironment.h b/dom/base/nsJSEnvironment.h
index 584eea93bd41..bb60b0e2b63b 100644
--- a/dom/base/nsJSEnvironment.h
+++ b/dom/base/nsJSEnvironment.h
@@ -94,6 +94,11 @@ class nsJSContext : public nsIScriptContext {
   static void RunNextCollectorTimer(
       JS::GCReason aReason,
       mozilla::TimeStamp aDeadline = mozilla::TimeStamp());
+  // If user has been idle and aDocShell is for an iframe being loaded in an
+  // already loaded top level docshell, this will run a CC or GC
+  // timer/runner if there is such pending.
+  static void MaybeRunNextCollectorSlice(nsIDocShell* aDocShell,
+                                         JS::GCReason aReason);
 
   // The GC should probably run soon, in the zone of object aObj (if given).
   static void PokeGC(JS::GCReason aReason, JSObject* aObj, uint32_t aDelay = 0);
diff --git a/parser/html/nsHtml5StreamParser.cpp b/parser/html/nsHtml5StreamParser.cpp
index 1bf036cb342b..76509f1ead58 100644
--- a/parser/html/nsHtml5StreamParser.cpp
+++ b/parser/html/nsHtml5StreamParser.cpp
@@ -999,6 +999,20 @@ void nsHtml5StreamParser::CommitLocalFileToEncoding() {
   }
 }
 
+class MaybeRunCollector : public Runnable {
+ public:
+  explicit MaybeRunCollector(nsIDocShell* aDocShell)
+      : Runnable("MaybeRunCollector"), mDocShell(aDocShell) {}
+
+  NS_IMETHOD Run() override {
+    nsJSContext::MaybeRunNextCollectorSlice(mDocShell,
+                                            JS::GCReason::HTML_PARSER);
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIDocShell> mDocShell;
+};
+
 nsresult nsHtml5StreamParser::OnStartRequest(nsIRequest* aRequest) {
   MOZ_RELEASE_ASSERT(STREAM_NOT_STARTED == mStreamState,
                      "Got OnStartRequest when the stream had already started.");
@@ -1150,6 +1164,16 @@ nsresult nsHtml5StreamParser::OnStartRequest(nsIRequest* aRequest) {
       do_QueryInterface(mRequest, &rv);
   if (threadRetargetableRequest) {
     rv = threadRetargetableRequest->RetargetDeliveryTo(mEventTarget);
+    if (NS_SUCCEEDED(rv)) {
+      // Parser thread should be now ready to get data from necko and parse it
+      // and main thread might have a chance to process a collector slice.
+      // We need to do this asynchronously so that necko may continue processing
+      // the request.
+      nsCOMPtr<nsIRunnable> runnable =
+          new MaybeRunCollector(mExecutor->GetDocument()->GetDocShell());
+      mozilla::SchedulerGroup::Dispatch(
+          mozilla::TaskCategory::GarbageCollection, runnable.forget());
+    }
   }
 
   if (NS_FAILED(rv)) {