--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1505,17 +1505,18 @@ nsIDocument::nsIDocument()
     mBoxObjectTable(nullptr),
     mCurrentOrientationAngle(0),
     mCurrentOrientationType(OrientationType::Portrait_primary),
     mServoRestyleRootDirtyBits(0),
     mThrowOnDynamicMarkupInsertionCounter(0),
     mIgnoreOpensDuringUnloadCounter(0),
     mNumTrackersFound(0),
     mNumTrackersBlocked(0),
-    mDocLWTheme(Doc_Theme_Uninitialized)
+    mDocLWTheme(Doc_Theme_Uninitialized),
+    mSavedResolution(1.0f)
 {
   SetIsInDocument();
   SetIsConnected(true);
 
   if (StaticPrefs::layout_css_use_counters_enabled()) {
     mStyleUseCounters.reset(Servo_UseCounters_Create());
   }
 }
@@ -11181,16 +11182,26 @@ nsIDocument::CleanupFullscreenState()
   // after bug 1195213.
   for (nsWeakPtr& weakPtr : Reversed(mFullscreenStack)) {
     if (nsCOMPtr<Element> element = do_QueryReferent(weakPtr)) {
       ClearFullscreenStateOnElement(element);
     }
   }
   mFullscreenStack.Clear();
   mFullscreenRoot = nullptr;
+
+  // Restore the zoom level that was in place prior to entering fullscreen.
+  if (nsIPresShell* shell = GetShell()) {
+    if (nsPresContext* context = shell->GetPresContext()) {
+      if (context->IsRootContentDocument()) {
+        shell->SetResolutionAndScaleTo(mSavedResolution);
+      }
+    }
+  }
+
   UpdateViewportScrollbarOverrideForFullscreen(this);
 }
 
 bool
 nsIDocument::FullscreenStackPush(Element* aElement)
 {
   NS_ASSERTION(aElement, "Must pass non-null to FullscreenStackPush()");
   Element* top = FullscreenStackTop();
@@ -11584,16 +11595,33 @@ nsIDocument::ApplyFullscreen(UniquePtr<F
   // Propagate up the document hierarchy, setting the fullscreen element as
   // the element's container in ancestor documents. This also sets the
   // appropriate css styles as well. Note we don't propagate down the
   // document hierarchy, the fullscreen element (or its container) is not
   // visible there. Stop when we reach the root document.
   nsIDocument* child = this;
   while (true) {
     child->SetFullscreenRoot(fullScreenRootDoc);
+
+    // When entering fullscreen, reset the RCD's zoom level to 1,
+    // otherwise the fullscreen content could be sized larger than the
+    // screen (since fullscreen is implemented using position:fixed and
+    // fixed elements are sized to the layout viewport).
+    // This also ensures that things like video controls aren't zoomed in
+    // when in fullscreen mode.
+    if (nsIPresShell* shell = child->GetShell()) {
+      if (nsPresContext* context = shell->GetPresContext()) {
+        if (context->IsRootContentDocument()) {
+          // Save the previous resolution so it can be restored.
+          child->mSavedResolution = shell->GetResolution();
+          shell->SetResolutionAndScaleTo(1.0f);
+        }
+      }
+    }
+
     NS_ASSERTION(child->GetFullscreenRoot() == fullScreenRootDoc,
         "Fullscreen root should be set!");
     if (child == fullScreenRootDoc) {
       break;
     }
     nsIDocument* parent = child->GetParentDocument();
     Element* element = parent->FindContentForSubDocument(child);
     if (static_cast<nsDocument*>(parent)->FullscreenStackPush(element)) {

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -4766,16 +4766,19 @@ protected:
   uint32_t mNumTrackersBlocked;
 
   mozilla::EnumSet<mozilla::Telemetry::LABELS_DOCUMENT_ANALYTICS_TRACKER_FASTBLOCKED>
     mTrackerBlockedReasons;
 
   // document lightweight theme for use with :-moz-lwtheme, :-moz-lwtheme-brighttext
   // and :-moz-lwtheme-darktext
   DocumentTheme                         mDocLWTheme;
+
+  // Pres shell resolution saved before entering fullscreen mode.
+  float mSavedResolution;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocument, NS_IDOCUMENT_IID)
 
 /**
  * mozAutoSubtreeModified batches DOM mutations so that a DOMSubtreeModified
  * event is dispatched, if necessary, when the outermost mozAutoSubtreeModified
  * object is deleted.