diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index 1289da1bcaa2..af61e3e01d8a 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -101,6 +101,7 @@
 #include "nsIContentIterator.h"
 #include "nsIDOMStyleSheet.h"
 #include "nsIStyleSheet.h"
+#include "nsIStyleSheetService.h"
 #include "nsContentPermissionHelper.h"
 #include "nsNetUtil.h"
 
@@ -3893,6 +3894,21 @@ nsDOMWindowUtils::LeaveChaosMode()
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::HasRuleProcessorUsedByMultipleStyleSets(uint32_t aSheetType,
+                                                          bool* aRetVal)
+{
+  MOZ_RELEASE_ASSERT(nsContentUtils::IsCallerChrome());
+
+  nsIPresShell* presShell = GetPresShell();
+  if (!presShell) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return presShell->HasRuleProcessorUsedByMultipleStyleSets(aSheetType,
+                                                            aRetVal);
+}
+
 NS_INTERFACE_MAP_BEGIN(nsTranslationNodeList)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
   NS_INTERFACE_MAP_ENTRY(nsITranslationNodeList)
diff --git a/dom/interfaces/base/nsIDOMWindowUtils.idl b/dom/interfaces/base/nsIDOMWindowUtils.idl
index eaf9f8129e4b..6a5cdb49f742 100644
--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -49,7 +49,7 @@ interface nsIJSRAIIHelper;
 interface nsIContentPermissionRequest;
 interface nsIObserver;
 
-[scriptable, uuid(336a8683-5626-4512-a3d5-ec280c13e5c2)]
+[scriptable, uuid(bbcb87fb-ce2e-4e05-906b-9258687664e2)]
 interface nsIDOMWindowUtils : nsISupports {
 
   /**
@@ -1857,6 +1857,15 @@ interface nsIDOMWindowUtils : nsISupports {
    * Decrease the chaos mode activation level. See enterChaosMode().
    */
   void leaveChaosMode();
+
+  /**
+   * Returns whether the document's style set's rule processor for the
+   * specified level of the cascade is shared by multiple style sets.
+   * (Used by tests to ensure that certain optimizations do not regress.)
+   *
+   * @param aSheetType One of the nsIStyleSheetService.*_SHEET constants.
+   */
+  bool hasRuleProcessorUsedByMultipleStyleSets(in unsigned long aSheetType);
 };
 
 [scriptable, uuid(c694e359-7227-4392-a138-33c0cc1f15a6)]
diff --git a/layout/base/nsIPresShell.h b/layout/base/nsIPresShell.h
index c15624856c67..5bf93287ca3d 100644
--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -1568,6 +1568,15 @@ public:
   // Whether we should assume all images are visible.
   virtual bool AssumeAllImagesVisible() = 0;
 
+  /**
+   * Returns whether the document's style set's rule processor for the
+   * specified level of the cascade is shared by multiple style sets.
+   *
+   * @param aSheetType One of the nsIStyleSheetService.*_SHEET constants.
+   */
+  nsresult HasRuleProcessorUsedByMultipleStyleSets(uint32_t aSheetType,
+                                                   bool* aRetVal);
+
   /**
    * Refresh observer management.
    */
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index 064891088275..663a498c3d2b 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -10894,3 +10894,27 @@ nsIPresShell::SyncWindowProperties(nsView* aView)
     nsContainerFrame::SyncWindowProperties(mPresContext, frame, aView, &rcx, 0);
   }
 }
+
+nsresult
+nsIPresShell::HasRuleProcessorUsedByMultipleStyleSets(uint32_t aSheetType,
+                                                      bool* aRetVal)
+{
+  nsStyleSet::sheetType type;
+  switch (aSheetType) {
+    case nsIStyleSheetService::AGENT_SHEET:
+      type = nsStyleSet::eAgentSheet;
+      break;
+    case nsIStyleSheetService::USER_SHEET:
+      type = nsStyleSet::eUserSheet;
+      break;
+    case nsIStyleSheetService::AUTHOR_SHEET:
+      type = nsStyleSet::eDocSheet;
+      break;
+    default:
+      MOZ_ASSERT(false, "unexpected aSheetType value");
+      return NS_ERROR_ILLEGAL_VALUE;
+  }
+
+  *aRetVal = mStyleSet->HasRuleProcessorUsedByMultipleStyleSets(type);
+  return NS_OK;
+}
diff --git a/layout/style/nsCSSRuleProcessor.h b/layout/style/nsCSSRuleProcessor.h
index 22015a7425e1..93fa68656dbf 100644
--- a/layout/style/nsCSSRuleProcessor.h
+++ b/layout/style/nsCSSRuleProcessor.h
@@ -183,6 +183,7 @@ public:
     mInRuleProcessorCache = aVal;
   }
   bool IsInRuleProcessorCache() const { return mInRuleProcessorCache; }
+  bool IsUsedByMultipleStyleSets() const { return mStyleSetRefCnt > 1; }
 
 #ifdef XP_WIN
   // Cached theme identifier for the moz-windows-theme media query.
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 633cc4adccb6..7b8268f7ba96 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -2467,3 +2467,15 @@ nsStyleSet::InitialStyleRule()
   }
   return mInitialStyleRule;
 }
+
+bool
+nsStyleSet::HasRuleProcessorUsedByMultipleStyleSets(sheetType aSheetType)
+{
+  MOZ_ASSERT(aSheetType < ArrayLength(mRuleProcessors));
+  if (!IsCSSSheetType(aSheetType) || !mRuleProcessors[aSheetType]) {
+    return false;
+  }
+  nsCSSRuleProcessor* rp =
+    static_cast<nsCSSRuleProcessor*>(mRuleProcessors[aSheetType].get());
+  return rp->IsUsedByMultipleStyleSets();
+}
diff --git a/layout/style/nsStyleSet.h b/layout/style/nsStyleSet.h
index 40c290d68bdb..717d5d00a71a 100644
--- a/layout/style/nsStyleSet.h
+++ b/layout/style/nsStyleSet.h
@@ -394,6 +394,8 @@ class nsStyleSet final
 
   nsIStyleRule* InitialStyleRule();
 
+  bool HasRuleProcessorUsedByMultipleStyleSets(sheetType aSheetType);
+
  private:
   nsStyleSet(const nsStyleSet& aCopy) = delete;
   nsStyleSet& operator=(const nsStyleSet& aCopy) = delete;
diff --git a/layout/style/test/browser.ini b/layout/style/test/browser.ini
index 42324bfc250a..0ddea4016fef 100644
--- a/layout/style/test/browser.ini
+++ b/layout/style/test/browser.ini
@@ -2,6 +2,8 @@
 support-files =
   bug453896_iframe.html
   media_queries_iframe.html
+  newtab_share_rule_processors.html
 
 [browser_bug453896.js]
 skip-if = e10s # Bug ?????? - test touches content (TypeError: doc.documentElement is null)
+[browser_newtab_share_rule_processors.js]
diff --git a/layout/style/test/browser_newtab_share_rule_processors.js b/layout/style/test/browser_newtab_share_rule_processors.js
new file mode 100644
index 000000000000..810f5f86e6b3
--- /dev/null
+++ b/layout/style/test/browser_newtab_share_rule_processors.js
@@ -0,0 +1,38 @@
+var theTab;
+var theBrowser;
+
+function listener(evt) {
+  if (evt.target == theBrowser.contentDocument) {
+    doTest();
+  }
+}
+
+function test() {
+  waitForExplicitFinish();
+  var testURL = getRootDirectory(gTestPath) + "newtab_share_rule_processors.html";
+  theTab = gBrowser.addTab(testURL);
+  theBrowser = gBrowser.getBrowserForTab(theTab);
+  theBrowser.addEventListener("load", listener, true);
+}
+
+function doTest() {
+  theBrowser.removeEventListener("load", listener, true);
+  var winUtils = theBrowser.contentWindow
+    .QueryInterface(Ci.nsIInterfaceRequestor)
+    .getInterface(Ci.nsIDOMWindowUtils);
+  // The initial set of agent-level sheets should have a rule processor that's
+  // also being used by another document.
+  ok(winUtils.hasRuleProcessorUsedByMultipleStyleSets(Ci.nsIStyleSheetService.AGENT_SHEET),
+     "agent sheet rule processor is used by multiple style sets");
+  // Document-level sheets currently never get shared rule processors.
+  ok(!winUtils.hasRuleProcessorUsedByMultipleStyleSets(Ci.nsIStyleSheetService.AUTHOR_SHEET),
+     "author sheet rule processor is not used by multiple style sets");
+  // Adding a unique style sheet to the agent level will cause it to have a
+  // rule processor that is unique.
+  theBrowser.contentWindow.wrappedJSObject.addAgentSheet();
+  ok(!winUtils.hasRuleProcessorUsedByMultipleStyleSets(Ci.nsIStyleSheetService.AGENT_SHEET),
+     "agent sheet rule processor is not used by multiple style sets after " +
+     "having a unique sheet added to it");
+  gBrowser.removeTab(theTab);
+  finish();
+}
diff --git a/layout/style/test/newtab_share_rule_processors.html b/layout/style/test/newtab_share_rule_processors.html
new file mode 100644
index 000000000000..bdfed1145b38
--- /dev/null
+++ b/layout/style/test/newtab_share_rule_processors.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<style>
+p { color: blue; }
+</style>
+<p>Hello</p>
+<script>
+var Cc = Components.classes;
+var Ci = Components.interfaces;
+
+var sss = Cc["@mozilla.org/content/style-sheet-service;1"]
+  .getService(Ci.nsIStyleSheetService);
+var io = Cc["@mozilla.org/network/io-service;1"]
+  .getService(Ci.nsIIOService);
+var winUtils = window.QueryInterface(Ci.nsIInterfaceRequestor)
+  .getInterface(Ci.nsIDOMWindowUtils);
+
+function addAgentSheet() {
+  var sheetURI = io.newURI("data:text/css,p{background-color:yellow}", null, null);
+  var sheet = sss.preloadSheet(sheetURI, Ci.nsIStyleSheetService.AGENT_SHEET);
+  winUtils.addSheet(sheet, Ci.nsIDOMWindowUtils.AGENT_SHEET);
+}
+</script>