--- a/dom/base/nsPlainTextSerializer.cpp
+++ b/dom/base/nsPlainTextSerializer.cpp
@@ -50,16 +50,19 @@ static const  char16_t kSPACE = ' ';
 static int32_t HeaderLevel(nsIAtom* aTag);
 static int32_t GetUnicharWidth(char16_t ucs);
 static int32_t GetUnicharStringWidth(const char16_t* pwcs, int32_t n);
 
 // Someday may want to make this non-const:
 static const uint32_t TagStackSize = 500;
 static const uint32_t OLStackSize = 100;
 
+static bool gPreferenceInitialized = false;
+static bool gAlwaysIncludeRuby = false;
+
 NS_IMPL_CYCLE_COLLECTING_ADDREF(nsPlainTextSerializer)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsPlainTextSerializer)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsPlainTextSerializer)
   NS_INTERFACE_MAP_ENTRY(nsIContentSerializer)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
 NS_INTERFACE_MAP_END
 
@@ -113,16 +116,22 @@ nsPlainTextSerializer::nsPlainTextSerial
 
   // initialize the OL stack, where numbers for ordered lists are kept
   mOLStack = new int32_t[OLStackSize];
   mOLStackIndex = 0;
 
   mULCount = 0;
 
   mIgnoredChildNodeLevel = 0;
+
+  if (!gPreferenceInitialized) {
+    Preferences::AddBoolVarCache(&gAlwaysIncludeRuby, PREF_ALWAYS_INCLUDE_RUBY,
+                                 true);
+    gPreferenceInitialized = true;
+  }
 }
 
 nsPlainTextSerializer::~nsPlainTextSerializer()
 {
   delete[] mTagStack;
   delete[] mOLStack;
   NS_WARNING_ASSERTION(mHeadLevel == 0, "Wrong head level!");
 }
@@ -185,17 +194,17 @@ nsPlainTextSerializer::Init(uint32_t aFl
     mHeaderStrategy =
       Preferences::GetInt(PREF_HEADER_STRATEGY, mHeaderStrategy);
   }
 
   // The pref is default inited to false in libpref, but we use true
   // as fallback value because we don't want to affect behavior in
   // other places which use this serializer currently.
   mWithRubyAnnotation =
-    Preferences::GetBool(PREF_ALWAYS_INCLUDE_RUBY, true) ||
+    gAlwaysIncludeRuby ||
     (mFlags & nsIDocumentEncoder::OutputRubyAnnotation);
 
   // XXX We should let the caller decide whether to do this or not
   mFlags &= ~nsIDocumentEncoder::OutputNoFramesContent;
 
   return NS_OK;
 }