--- a/layout/style/nsCSSPseudoClassList.h
+++ b/layout/style/nsCSSPseudoClassList.h
@@ -117,16 +117,19 @@ CSS_PSEUDO_CLASS(mozLWThemeDarkText, ":-
 
 // Matches anything when the containing window is inactive
 CSS_PSEUDO_CLASS(mozWindowInactive, ":-moz-window-inactive", 0, "")
 
 // Matches any table elements that have a nonzero border attribute,
 // according to HTML integer attribute parsing rules.
 CSS_PSEUDO_CLASS(mozTableBorderNonzero, ":-moz-table-border-nonzero", 0, "")
 
+// Matches HTML frame/iframe elements which are mozbrowser.
+CSS_PSEUDO_CLASS(mozBrowserFrame, ":-moz-browser-frame", 0, "")
+
 // Matches whatever the contextual reference elements are for the
 // matching operation.
 CSS_PSEUDO_CLASS(scope, ":scope", 0, "layout.css.scope-pseudo.enabled")
 
 // :not needs to come at the end of the non-bit pseudo-class list, since
 // it doesn't actually get directly matched on in SelectorMatches.
 CSS_PSEUDO_CLASS(notPseudo, ":not", 0, "")
 

--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -51,16 +51,17 @@
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/EventStates.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/LookAndFeel.h"
 #include "mozilla/Likely.h"
 #include "mozilla/TypedEnumBits.h"
 #include "RuleProcessorCache.h"
 #include "nsIDOMMutationEvent.h"
+#include "nsIMozBrowserFrame.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 #define VISITED_PSEUDO_PREF "layout.css.visited_links_enabled"
 
 static bool gSupportVisitedPseudo = true;
 
@@ -2151,16 +2152,27 @@ static bool SelectorMatches(Element* aEl
           if (!val ||
               (val->Type() == nsAttrValue::eInteger &&
                val->GetIntegerValue() == 0)) {
             return false;
           }
         }
         break;
 
+      case nsCSSPseudoClasses::ePseudoClass_mozBrowserFrame:
+        {
+          nsCOMPtr<nsIMozBrowserFrame>
+            browserFrame = do_QueryInterface(aElement);
+          if (!browserFrame ||
+              !browserFrame->GetReallyIsBrowserOrApp()) {
+            return false;
+          }
+        }
+        break;
+
       case nsCSSPseudoClasses::ePseudoClass_dir:
         {
           if (aDependence) {
             EventStates states
               = sPseudoClassStateDependences[pseudoClass->mType];
             if (aNodeMatchContext.mStateMask.HasAtLeastOneOfStates(states)) {
               *aDependence = true;
               return false;