diff --git a/extensions/universalchardet/src/base/CharDistribution.cpp b/extensions/universalchardet/src/base/CharDistribution.cpp
index ce72d3b93d72..4817c0e2f3a4 100644
--- a/extensions/universalchardet/src/base/CharDistribution.cpp
+++ b/extensions/universalchardet/src/base/CharDistribution.cpp
@@ -49,12 +49,13 @@
 #define MINIMUM_DATA_THRESHOLD  4
 
 //return confidence base on received data
-float CharDistributionAnalysis::GetConfidence()
+float CharDistributionAnalysis::GetConfidence(PRBool aIsPreferredLanguage)
 { 
   //if we didn't receive any character in our consideration range, or the
   // number of frequent characters is below the minimum threshold, return
   // negative answer
-  if (mTotalChars <= 0 || mFreqChars <= MINIMUM_DATA_THRESHOLD)
+  if (mTotalChars <= 0 ||
+      !aIsPreferredLanguage && mFreqChars <= MINIMUM_DATA_THRESHOLD)
     return SURE_NO;
 
   if (mTotalChars != mFreqChars) {
diff --git a/extensions/universalchardet/src/base/CharDistribution.h b/extensions/universalchardet/src/base/CharDistribution.h
index dbc9e564876c..99f338ca6579 100644
--- a/extensions/universalchardet/src/base/CharDistribution.h
+++ b/extensions/universalchardet/src/base/CharDistribution.h
@@ -71,7 +71,7 @@ public:
   }
 
   //return confidence base on existing data
-  float GetConfidence();
+  float GetConfidence(PRBool aIsPreferredLanguage);
 
   //Reset analyser, clear any state 
   void      Reset(void) 
diff --git a/extensions/universalchardet/src/base/JpCntx.cpp b/extensions/universalchardet/src/base/JpCntx.cpp
index 812c3bf9633e..8375fdcfc7e0 100644
--- a/extensions/universalchardet/src/base/JpCntx.cpp
+++ b/extensions/universalchardet/src/base/JpCntx.cpp
@@ -181,10 +181,10 @@ void JapaneseContextAnalysis::Reset(void)
 }
 #define DONT_KNOW (float)-1
 
-float  JapaneseContextAnalysis::GetConfidence()
+float  JapaneseContextAnalysis::GetConfidence(PRBool aIsPreferredLanguage)
 {
   //This is just one way to calculate confidence. It works well for me.
-  if (mTotalRel > MINIMUM_DATA_THRESHOLD)
+  if (aIsPreferredLanguage || mTotalRel > MINIMUM_DATA_THRESHOLD)
     return ((float)(mTotalRel - mRelSample[0]))/mTotalRel;
   else 
     return (float)DONT_KNOW;
diff --git a/extensions/universalchardet/src/base/JpCntx.h b/extensions/universalchardet/src/base/JpCntx.h
index 1760b752ac78..e86445932304 100644
--- a/extensions/universalchardet/src/base/JpCntx.h
+++ b/extensions/universalchardet/src/base/JpCntx.h
@@ -74,7 +74,7 @@ public:
     mLastCharOrder = order;
   }
 
-  float GetConfidence();
+  float GetConfidence(PRBool aIsPreferredLanguage);
   void      Reset(void);
   void      SetOpion(){}
   PRBool GotEnoughData() {return mTotalRel > ENOUGH_REL_THRESHOLD;}
diff --git a/extensions/universalchardet/src/base/nsBig5Prober.cpp b/extensions/universalchardet/src/base/nsBig5Prober.cpp
index 7288eb15966a..aee5cd483e2a 100644
--- a/extensions/universalchardet/src/base/nsBig5Prober.cpp
+++ b/extensions/universalchardet/src/base/nsBig5Prober.cpp
@@ -81,7 +81,7 @@ nsProbingState nsBig5Prober::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsBig5Prober::GetConfidence(void)
 {
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (float)distribCf;
 }
diff --git a/extensions/universalchardet/src/base/nsBig5Prober.h b/extensions/universalchardet/src/base/nsBig5Prober.h
index 188c6d7618f1..5ae357643fc8 100644
--- a/extensions/universalchardet/src/base/nsBig5Prober.h
+++ b/extensions/universalchardet/src/base/nsBig5Prober.h
@@ -44,8 +44,10 @@
 
 class nsBig5Prober: public nsCharSetProber {
 public:
-  nsBig5Prober(void){mCodingSM = new nsCodingStateMachine(&Big5SMModel);
-                      Reset();}
+  nsBig5Prober(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage) 
+  {mCodingSM = new nsCodingStateMachine(&Big5SMModel); 
+    Reset();}
   virtual ~nsBig5Prober(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "Big5";}
@@ -63,6 +65,7 @@ protected:
   //Big5ContextAnalysis mContextAnalyser;
   Big5DistributionAnalysis mDistributionAnalyser;
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 
 };
 
diff --git a/extensions/universalchardet/src/base/nsEUCJPProber.cpp b/extensions/universalchardet/src/base/nsEUCJPProber.cpp
index 68a8b519aca5..35387dfddacd 100644
--- a/extensions/universalchardet/src/base/nsEUCJPProber.cpp
+++ b/extensions/universalchardet/src/base/nsEUCJPProber.cpp
@@ -91,8 +91,8 @@ nsProbingState nsEUCJPProber::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsEUCJPProber::GetConfidence(void)
 {
-  float contxtCf = mContextAnalyser.GetConfidence();
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float contxtCf = mContextAnalyser.GetConfidence(mIsPreferredLanguage);
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (contxtCf > distribCf ? contxtCf : distribCf);
 }
diff --git a/extensions/universalchardet/src/base/nsEUCJPProber.h b/extensions/universalchardet/src/base/nsEUCJPProber.h
index 05974e214c35..a7a2f51471f3 100644
--- a/extensions/universalchardet/src/base/nsEUCJPProber.h
+++ b/extensions/universalchardet/src/base/nsEUCJPProber.h
@@ -50,8 +50,10 @@
 
 class nsEUCJPProber: public nsCharSetProber {
 public:
-  nsEUCJPProber(void){mCodingSM = new nsCodingStateMachine(&EUCJPSMModel);
-                      Reset();}
+  nsEUCJPProber(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage)
+  {mCodingSM = new nsCodingStateMachine(&EUCJPSMModel);
+    Reset();}
   virtual ~nsEUCJPProber(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "EUC-JP";}
@@ -68,6 +70,7 @@ protected:
   EUCJPDistributionAnalysis mDistributionAnalyser;
 
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 };
 
 
diff --git a/extensions/universalchardet/src/base/nsEUCKRProber.cpp b/extensions/universalchardet/src/base/nsEUCKRProber.cpp
index 7d278ad8fd64..396b09527e5f 100644
--- a/extensions/universalchardet/src/base/nsEUCKRProber.cpp
+++ b/extensions/universalchardet/src/base/nsEUCKRProber.cpp
@@ -84,7 +84,7 @@ nsProbingState nsEUCKRProber::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsEUCKRProber::GetConfidence(void)
 {
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (float)distribCf;
 }
diff --git a/extensions/universalchardet/src/base/nsEUCKRProber.h b/extensions/universalchardet/src/base/nsEUCKRProber.h
index 959584cd8431..8e0998460280 100644
--- a/extensions/universalchardet/src/base/nsEUCKRProber.h
+++ b/extensions/universalchardet/src/base/nsEUCKRProber.h
@@ -44,8 +44,11 @@
 
 class nsEUCKRProber: public nsCharSetProber {
 public:
-  nsEUCKRProber(void){mCodingSM = new nsCodingStateMachine(&EUCKRSMModel);
-                      Reset();}
+  nsEUCKRProber(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage)
+  {mCodingSM = new nsCodingStateMachine(&EUCKRSMModel);
+    Reset();
+  }
   virtual ~nsEUCKRProber(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "EUC-KR";}
@@ -63,6 +66,7 @@ protected:
   //EUCKRContextAnalysis mContextAnalyser;
   EUCKRDistributionAnalysis mDistributionAnalyser;
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 
 };
 
diff --git a/extensions/universalchardet/src/base/nsEUCTWProber.cpp b/extensions/universalchardet/src/base/nsEUCTWProber.cpp
index ef2bcf50c336..710e413eb1db 100644
--- a/extensions/universalchardet/src/base/nsEUCTWProber.cpp
+++ b/extensions/universalchardet/src/base/nsEUCTWProber.cpp
@@ -84,7 +84,7 @@ nsProbingState nsEUCTWProber::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsEUCTWProber::GetConfidence(void)
 {
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (float)distribCf;
 }
diff --git a/extensions/universalchardet/src/base/nsEUCTWProber.h b/extensions/universalchardet/src/base/nsEUCTWProber.h
index cd4a4aff482c..911d50b03992 100644
--- a/extensions/universalchardet/src/base/nsEUCTWProber.h
+++ b/extensions/universalchardet/src/base/nsEUCTWProber.h
@@ -44,8 +44,10 @@
 
 class nsEUCTWProber: public nsCharSetProber {
 public:
-  nsEUCTWProber(void){mCodingSM = new nsCodingStateMachine(&EUCTWSMModel);
-                      Reset();}
+  nsEUCTWProber(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage)
+  {mCodingSM = new nsCodingStateMachine(&EUCTWSMModel);
+    Reset();}
   virtual ~nsEUCTWProber(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "x-euc-tw";}
@@ -63,6 +65,7 @@ protected:
   //EUCTWContextAnalysis mContextAnalyser;
   EUCTWDistributionAnalysis mDistributionAnalyser;
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 
 };
 
diff --git a/extensions/universalchardet/src/base/nsGB2312Prober.cpp b/extensions/universalchardet/src/base/nsGB2312Prober.cpp
index f05b6d7fa965..95374c3122e6 100644
--- a/extensions/universalchardet/src/base/nsGB2312Prober.cpp
+++ b/extensions/universalchardet/src/base/nsGB2312Prober.cpp
@@ -89,7 +89,7 @@ nsProbingState nsGB18030Prober::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsGB18030Prober::GetConfidence(void)
 {
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (float)distribCf;
 }
diff --git a/extensions/universalchardet/src/base/nsGB2312Prober.h b/extensions/universalchardet/src/base/nsGB2312Prober.h
index be01907886e2..4bdac3bbe032 100644
--- a/extensions/universalchardet/src/base/nsGB2312Prober.h
+++ b/extensions/universalchardet/src/base/nsGB2312Prober.h
@@ -46,8 +46,10 @@
 
 class nsGB18030Prober: public nsCharSetProber {
 public:
-  nsGB18030Prober(void){mCodingSM = new nsCodingStateMachine(&GB18030SMModel);
-                      Reset();}
+  nsGB18030Prober(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage)
+  {mCodingSM = new nsCodingStateMachine(&GB18030SMModel);
+    Reset();}
   virtual ~nsGB18030Prober(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "gb18030";}
@@ -65,6 +67,7 @@ protected:
   //GB2312ContextAnalysis mContextAnalyser;
   GB2312DistributionAnalysis mDistributionAnalyser;
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 
 };
 
diff --git a/extensions/universalchardet/src/base/nsMBCSGroupProber.cpp b/extensions/universalchardet/src/base/nsMBCSGroupProber.cpp
index ffb80cfe5a95..b07234db5ff0 100644
--- a/extensions/universalchardet/src/base/nsMBCSGroupProber.cpp
+++ b/extensions/universalchardet/src/base/nsMBCSGroupProber.cpp
@@ -63,17 +63,17 @@ nsMBCSGroupProber::nsMBCSGroupProber(PRUint32 aLanguageFilter)
   mProbers[0] = new nsUTF8Prober();
   if (aLanguageFilter & NS_FILTER_JAPANESE) 
   {
-    mProbers[1] = new nsSJISProber();
-    mProbers[2] = new nsEUCJPProber();
+    mProbers[1] = new nsSJISProber(aLanguageFilter == NS_FILTER_JAPANESE);
+    mProbers[2] = new nsEUCJPProber(aLanguageFilter == NS_FILTER_JAPANESE);
   }
   if (aLanguageFilter & NS_FILTER_CHINESE_SIMPLIFIED)
-    mProbers[3] = new nsGB18030Prober();
+    mProbers[3] = new nsGB18030Prober(aLanguageFilter == NS_FILTER_CHINESE_SIMPLIFIED);
   if (aLanguageFilter & NS_FILTER_KOREAN)
-    mProbers[4] = new nsEUCKRProber();
+    mProbers[4] = new nsEUCKRProber(aLanguageFilter == NS_FILTER_KOREAN);
   if (aLanguageFilter & NS_FILTER_CHINESE_TRADITIONAL) 
   {
-    mProbers[5] = new nsBig5Prober();
-    mProbers[6] = new nsEUCTWProber();
+    mProbers[5] = new nsBig5Prober(aLanguageFilter == NS_FILTER_CHINESE_TRADITIONAL);
+    mProbers[6] = new nsEUCTWProber(aLanguageFilter == NS_FILTER_CHINESE_TRADITIONAL);
   }
   Reset();
 }
diff --git a/extensions/universalchardet/src/base/nsSJISProber.cpp b/extensions/universalchardet/src/base/nsSJISProber.cpp
index eb39d91d2784..5b7e7fddc795 100644
--- a/extensions/universalchardet/src/base/nsSJISProber.cpp
+++ b/extensions/universalchardet/src/base/nsSJISProber.cpp
@@ -90,8 +90,8 @@ nsProbingState nsSJISProber::HandleData(const char* aBuf, PRUint32 aLen)
 
 float nsSJISProber::GetConfidence(void)
 {
-  float contxtCf = mContextAnalyser.GetConfidence();
-  float distribCf = mDistributionAnalyser.GetConfidence();
+  float contxtCf = mContextAnalyser.GetConfidence(mIsPreferredLanguage);
+  float distribCf = mDistributionAnalyser.GetConfidence(mIsPreferredLanguage);
 
   return (contxtCf > distribCf ? contxtCf : distribCf);
 }
diff --git a/extensions/universalchardet/src/base/nsSJISProber.h b/extensions/universalchardet/src/base/nsSJISProber.h
index 759cca606cf5..1efb6e3d639e 100644
--- a/extensions/universalchardet/src/base/nsSJISProber.h
+++ b/extensions/universalchardet/src/base/nsSJISProber.h
@@ -51,8 +51,10 @@
 
 class nsSJISProber: public nsCharSetProber {
 public:
-  nsSJISProber(void){mCodingSM = new nsCodingStateMachine(&SJISSMModel);
-                      Reset();}
+  nsSJISProber(PRBool aIsPreferredLanguage)
+    :mIsPreferredLanguage(aIsPreferredLanguage)
+  {mCodingSM = new nsCodingStateMachine(&SJISSMModel);
+    Reset();}
   virtual ~nsSJISProber(void){delete mCodingSM;}
   nsProbingState HandleData(const char* aBuf, PRUint32 aLen);
   const char* GetCharSetName() {return "Shift_JIS";}
@@ -69,6 +71,7 @@ protected:
   SJISDistributionAnalysis mDistributionAnalyser;
 
   char mLastChar[2];
+  PRBool mIsPreferredLanguage;
 
 };
 
diff --git a/extensions/universalchardet/tests/Makefile.in b/extensions/universalchardet/tests/Makefile.in
index e900286a4ab7..27e45f2d8ba2 100644
--- a/extensions/universalchardet/tests/Makefile.in
+++ b/extensions/universalchardet/tests/Makefile.in
@@ -83,6 +83,9 @@ _TEST_FILES =	\
 		test_bug426271-euc-jp.html \
 		bug426271_text-utf-8.html \
 		test_bug426271-utf-8.html \
+		bug431054_text.html \
+		test_bug431054.html \
+		test_bug431054-japanese.html \
 		$(NULL)
 
 libs:: $(_TEST_FILES)
diff --git a/extensions/universalchardet/tests/bug431054_text.html b/extensions/universalchardet/tests/bug431054_text.html
new file mode 100644
index 000000000000..aa88a4e1d2d1
--- /dev/null
+++ b/extensions/universalchardet/tests/bug431054_text.html
@@ -0,0 +1,5 @@
+<html>
+<body>
+¥í¥°¥¤¥ó
+</body>
+</html>
diff --git a/extensions/universalchardet/tests/test_bug306272.html b/extensions/universalchardet/tests/test_bug306272.html
index 9f7643fc53a7..296b34cfd6f0 100644
--- a/extensions/universalchardet/tests/test_bug306272.html
+++ b/extensions/universalchardet/tests/test_bug306272.html
@@ -22,7 +22,6 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=306272
 CharsetDetectionTests("bug306272_text.html",
 		      "UTF-8",
 		      new Array("ja_parallel_state_machine",
-				"ko_parallel_state_machine",
 				"zh_parallel_state_machine",
 				"zhtw_parallel_state_machine",
 				"zhcn_parallel_state_machine",
diff --git a/extensions/universalchardet/tests/test_bug431054-japanese.html b/extensions/universalchardet/tests/test_bug431054-japanese.html
new file mode 100644
index 000000000000..537f30b298a9
--- /dev/null
+++ b/extensions/universalchardet/tests/test_bug431054-japanese.html
@@ -0,0 +1,28 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=431054
+-->
+<head>
+  <title>Test for Bug 431054</title>
+  <script type="text/javascript" src="/MochiKit/MochiKit.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="CharsetDetectionTests.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=426271">Mozilla Bug 431054</a>
+<p id="display"></p>
+<div id="content" style="display: none">  
+</div>
+<iframe id="testframe"></iframe>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+/** Test for Bug 431054 **/
+CharsetDetectionTests("bug431054_text.html",
+		      "EUC-JP",
+		      new Array("ja_parallel_state_machine"));
+</script>
+</pre>
+</body>
+</html>
diff --git a/extensions/universalchardet/tests/test_bug431054.html b/extensions/universalchardet/tests/test_bug431054.html
new file mode 100644
index 000000000000..4a8ac0f4d890
--- /dev/null
+++ b/extensions/universalchardet/tests/test_bug431054.html
@@ -0,0 +1,32 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=431054
+-->
+<head>
+  <title>Test for Bug 431054</title>
+  <script type="text/javascript" src="/MochiKit/MochiKit.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="CharsetDetectionTests.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=426271">Mozilla Bug 431054</a>
+<p id="display"></p>
+<div id="content" style="display: none">  
+</div>
+<iframe id="testframe"></iframe>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+/** Test for Bug 431054 **/
+CharsetDetectionTests("bug431054_text.html",
+		      "windows-1252",
+		      new Array("zhtw_parallel_state_machine",
+				"zhcn_parallel_state_machine",
+				"zh_parallel_state_machine",
+				"cjk_parallel_state_machine",
+				"universal_charset_detector"));
+</script>
+</pre>
+</body>
+</html>