diff --git a/mozilla/gfx/thebes/public/gfxFont.h b/mozilla/gfx/thebes/public/gfxFont.h
index 03ef48947ea..c20f6f4d522 100644
--- a/mozilla/gfx/thebes/public/gfxFont.h
+++ b/mozilla/gfx/thebes/public/gfxFont.h
@@ -468,7 +468,8 @@ public:
          */
         TEXT_ENABLE_HYPHEN_BREAKS    = 0x0040,
         /**
-         * When set, the text has no characters above 255.
+         * When set, the text has no characters above 255 and it is stored
+         * in the textrun in 8-bit format.
          */
         TEXT_IS_8BIT                 = 0x0080,
         /**
@@ -485,9 +486,10 @@ public:
          */
         TEXT_NEED_BOUNDING_BOX       = 0x0200,
         /**
-         * When set, ligatures are disabled.
+         * When set, optional ligatures are disabled. Ligatures that are
+         * required for legible text should still be enabled.
          */
-        TEXT_DISABLE_LIGATURES       = 0x0400
+        TEXT_DISABLE_OPTIONAL_LIGATURES = 0x0400
     };
 
     /**
diff --git a/mozilla/gfx/thebes/src/gfxAtsuiFonts.cpp b/mozilla/gfx/thebes/src/gfxAtsuiFonts.cpp
index a665dffaae2..ee413c4fef5 100644
--- a/mozilla/gfx/thebes/src/gfxAtsuiFonts.cpp
+++ b/mozilla/gfx/thebes/src/gfxAtsuiFonts.cpp
@@ -936,10 +936,9 @@ AddGlyphRun(gfxTextRun *aRun, gfxAtsuiFont *aFont, PRUint32 aOffset)
 }
 
 static void
-DisableLigaturesInStyle(ATSUStyle aStyle)
+DisableOptionalLigaturesInStyle(ATSUStyle aStyle)
 {
-    static ATSUFontFeatureType selectors[9] = {
-        kRequiredLigaturesOffSelector,
+    static ATSUFontFeatureType selectors[] = {
         kCommonLigaturesOffSelector,
         kRareLigaturesOffSelector,
         kLogosOffSelector,
@@ -957,7 +956,6 @@ DisableLigaturesInStyle(ATSUStyle aStyle)
         kLigaturesType,
         kLigaturesType,
         kLigaturesType,
-        kLigaturesType,
         kLigaturesType
     };
     ATSUSetFontFeatures(aStyle, NS_ARRAY_LENGTH(selectors), types, selectors);
@@ -1027,11 +1025,11 @@ gfxAtsuiFontGroup::InitTextRun(gfxTextRun *aRun,
     printf("%p(%s) TEXTRUN \"%s\" ENDTEXTRUN\n", this, families.get(), str.get());
 #endif
 
-    if (aRun->GetFlags() & TEXT_DISABLE_LIGATURES) {
+    if (aRun->GetFlags() & TEXT_DISABLE_OPTIONAL_LIGATURES) {
         status = ATSUCreateAndCopyStyle(mainStyle, &mainStyle);
         if (status == noErr) {
             stylesToDispose.AppendElement(mainStyle);
-            DisableLigaturesInStyle(mainStyle);
+            DisableOptionalLigaturesInStyle(mainStyle);
         }
     }
 
diff --git a/mozilla/gfx/thebes/src/gfxTextRunWordCache.cpp b/mozilla/gfx/thebes/src/gfxTextRunWordCache.cpp
index 21d81a0f57d..70f7aef204e 100644
--- a/mozilla/gfx/thebes/src/gfxTextRunWordCache.cpp
+++ b/mozilla/gfx/thebes/src/gfxTextRunWordCache.cpp
@@ -37,7 +37,6 @@
 
 #include "gfxTextRunWordCache.h"
 
-
 /**
  * Cache individual "words" (strings delimited by white-space or white-space-like
  * characters that don't involve kerning or ligatures) in textruns.
@@ -93,6 +92,19 @@ protected:
         PRUint32     mStringHash;
         PRPackedBool mIsDoubleByteText;
         PRPackedBool mIsRTL;
+        PRPackedBool mEnabledOptionalLigatures;
+        
+        CacheHashKey(gfxTextRun *aBaseTextRun, void *aFontOrGroup,
+                     PRUint32 aStart, PRUint32 aLength, PRUint32 aHash)
+            : mFontOrGroup(aFontOrGroup), mString(aBaseTextRun->GetTextAt(aStart)),
+              mLength(aLength),
+              mAppUnitsPerDevUnit(aBaseTextRun->GetAppUnitsPerDevUnit()),
+              mStringHash(aHash),
+              mIsDoubleByteText((aBaseTextRun->GetFlags() & gfxTextRunFactory::TEXT_IS_8BIT) == 0),
+              mIsRTL(aBaseTextRun->IsRightToLeft()),
+              mEnabledOptionalLigatures((aBaseTextRun->GetFlags() & gfxTextRunFactory::TEXT_DISABLE_OPTIONAL_LIGATURES) == 0)
+        {
+        }
     };
 
     class CacheHashEntry : public PLDHashEntryHdr {
@@ -220,12 +232,7 @@ TextRunWordCache::LookupWord(gfxTextRun *aTextRun, gfxFont *aFirstFont,
     if (aEnd <= aStart)
         return PR_TRUE;
 
-    PRUint32 length = aEnd - aStart;
-    CacheHashKey key =
-        { aFirstFont, aTextRun->GetTextAt(aStart),
-          length, aTextRun->GetAppUnitsPerDevUnit(), aHash,
-          (aTextRun->GetFlags() & gfxTextRunFactory::TEXT_IS_8BIT) == 0,
-          aTextRun->IsRightToLeft() };
+    CacheHashKey key(aTextRun, aFirstFont, aStart, aEnd - aStart, aHash);
     CacheHashEntry *fontEntry = mCache.PutEntry(key);
     if (!fontEntry)
         return PR_FALSE;
@@ -304,11 +311,8 @@ TextRunWordCache::FinishTextRun(gfxTextRun *aTextRun, gfxTextRun *aNewRun,
             // keyed off the fontgroup.
             if (!aSuccessful ||
                 GetWordFontOrGroup(aNewRun, word->mSourceOffset, word->mLength) != font) {
-                CacheHashKey key =
-                    { font, aTextRun->GetTextAt(word->mDestOffset),
-                      word->mLength, aTextRun->GetAppUnitsPerDevUnit(), word->mHash,
-                      (aTextRun->GetFlags() & gfxTextRunFactory::TEXT_IS_8BIT) == 0,
-                      aTextRun->IsRightToLeft() };
+                CacheHashKey key(aTextRun, font, word->mDestOffset, word->mLength,
+                                 word->mHash);
                 NS_ASSERTION(mCache.GetEntry(key),
                              "This entry should have been added previously!");
                 mCache.RemoveEntry(key);
@@ -506,18 +510,15 @@ TextRunWordCache::RemoveWord(gfxTextRun *aTextRun, PRUint32 aStart,
         return;
 
     PRUint32 length = aEnd - aStart;
-    CacheHashKey key =
-        { GetWordFontOrGroup(aTextRun, aStart, length), aTextRun->GetTextAt(aStart),
-          length, aTextRun->GetAppUnitsPerDevUnit(), aHash,
-          (aTextRun->GetFlags() & gfxTextRunFactory::TEXT_IS_8BIT) == 0,
-          aTextRun->IsRightToLeft() };
+    CacheHashKey key(aTextRun, GetWordFontOrGroup(aTextRun, aStart, length),
+                     aStart, length, aHash);
     CacheHashEntry *entry = mCache.GetEntry(key);
     if (entry && entry->mTextRun == aTextRun) {
         // XXX would like to use RawRemoveEntry here plus some extra method
         // that conditionally shrinks the hashtable
         mCache.RemoveEntry(key);
         PR_LOG(gWordCacheLog, PR_LOG_DEBUG, ("%p(%d-%d,%d): removed using %s",
-            aTextRun, aStart, aEnd - aStart, aHash,
+            aTextRun, aStart, length, aHash,
             key.mFontOrGroup == aTextRun->GetFontGroup() ? "fontgroup" : "font"));
     }
 }
@@ -584,7 +585,8 @@ TextRunWordCache::CacheHashEntry::KeyEquals(const KeyTypePointer aKey) const
     if (!IsWordEnd(mTextRun, mWordOffset + length) ||
         GetFontOrGroup(fontGroup, mHashedByFont) != aKey->mFontOrGroup ||
         aKey->mAppUnitsPerDevUnit != mTextRun->GetAppUnitsPerDevUnit() ||
-        aKey->mIsRTL != mTextRun->IsRightToLeft())
+        aKey->mIsRTL != mTextRun->IsRightToLeft() ||
+        aKey->mEnabledOptionalLigatures != ((mTextRun->GetFlags() & gfxTextRunFactory::TEXT_DISABLE_OPTIONAL_LIGATURES) == 0))
         return PR_FALSE;
 
     if (mTextRun->GetFlags() & gfxFontGroup::TEXT_IS_8BIT) {
@@ -606,7 +608,7 @@ PLDHashNumber
 TextRunWordCache::CacheHashEntry::HashKey(const KeyTypePointer aKey)
 {
     return aKey->mStringHash + (long)aKey->mFontOrGroup + aKey->mAppUnitsPerDevUnit +
-        aKey->mIsDoubleByteText + aKey->mIsRTL * 2;
+        aKey->mIsDoubleByteText + aKey->mIsRTL*2 + aKey->mEnabledOptionalLigatures*4;
 }
 
 static TextRunWordCache *gTextRunWordCache = nsnull;
diff --git a/mozilla/layout/generic/nsTextFrameThebes.cpp b/mozilla/layout/generic/nsTextFrameThebes.cpp
index d293b947d1f..e010ddef44b 100644
--- a/mozilla/layout/generic/nsTextFrameThebes.cpp
+++ b/mozilla/layout/generic/nsTextFrameThebes.cpp
@@ -1285,7 +1285,7 @@ static nscoord StyleToCoord(const nsStyleCoord& aCoord)
 }
 
 static PRBool
-ShouldDisableLigatures(const nsStyleText* aTextStyle)
+ShouldDisableOptionalLigatures(const nsStyleText* aTextStyle)
 {
   return StyleToCoord(aTextStyle->mLetterSpacing) != 0;
 }
@@ -1322,7 +1322,7 @@ BuildTextRunsScanner::ContinueTextRunAcrossFrames(nsTextFrame* aFrame1, nsTextFr
     return PR_TRUE;
   return sc1->GetStyleFont()->mFont.BaseEquals(sc2->GetStyleFont()->mFont) &&
     sc1->GetStyleVisibility()->mLangGroup == sc2->GetStyleVisibility()->mLangGroup &&
-    ShouldDisableLigatures(textStyle1) == ShouldDisableLigatures(sc2->GetStyleText());
+    ShouldDisableOptionalLigatures(textStyle1) == ShouldDisableOptionalLigatures(sc2->GetStyleText());
 }
 
 void BuildTextRunsScanner::ScanFrame(nsIFrame* aFrame)
@@ -1708,8 +1708,8 @@ BuildTextRunsScanner::BuildTextRunForFrames(void* aTextBuffer)
   if (mTrimNextRunLeadingWhitespace) {
     textFlags |= nsTextFrameUtils::TEXT_TRAILING_WHITESPACE;
   }
-  if (ShouldDisableLigatures(firstFrame->GetStyleText())) {
-    textFlags |= gfxTextRunFactory::TEXT_DISABLE_LIGATURES;
+  if (ShouldDisableOptionalLigatures(firstFrame->GetStyleText())) {
+    textFlags |= gfxTextRunFactory::TEXT_DISABLE_OPTIONAL_LIGATURES;
   }
  
   gfxSkipChars skipChars;