diff --git a/gfx/thebes/gfxFontEntry.cpp b/gfx/thebes/gfxFontEntry.cpp
index 4d8d72ec9..a333d0bfc 100644
--- a/gfx/thebes/gfxFontEntry.cpp
+++ b/gfx/thebes/gfxFontEntry.cpp
@@ -1720,6 +1720,8 @@ gfxFontFamily::ReadFaceNames(gfxPlatformFontList *aPlatformFontList,
     }
 #endif
 
+#ifdef __LP64__
+// Can't run on 10.4-10.6.
     if (!mOtherFamilyNamesInitialized &&
         aFontInfoData &&
         aFontInfoData->mLoadOtherNames &&
@@ -1742,6 +1744,7 @@ gfxFontFamily::ReadFaceNames(gfxPlatformFontList *aPlatformFontList,
         (mFaceNamesInitialized || !aNeedFullnamePostscriptNames)) {
         return;
     }
+#endif
 
     FindStyleVariations(aFontInfoData);
 
diff --git a/gfx/thebes/gfxMacFont.cpp b/gfx/thebes/gfxMacFont.cpp
index 7f45cc077..dcf394790 100644
--- a/gfx/thebes/gfxMacFont.cpp
+++ b/gfx/thebes/gfxMacFont.cpp
@@ -223,18 +223,18 @@ gfxMacFont::InitMetrics()
 
 {
     ByteCount tableSize;
-    if (::ATSFontGetTable(myATSFont,
+    if (MOZ_UNLIKELY(::ATSFontGetTable(myATSFont,
 		TRUETYPE_TAG('h','e','a','d'),
-		0, 0, NULL, &tableSize) != noErr)
+		0, 0, NULL, &tableSize) != noErr))
 	return;
     CFMutableDataRef data = ::CFDataCreateMutable(kCFAllocatorDefault,
 	tableSize);
-    if (!data) return;
+    if (MOZ_UNLIKELY(!data)) return;
     ::CFDataIncreaseLength(data, tableSize);
-    if (::ATSFontGetTable(myATSFont,
+    if (MOZ_UNLIKELY(::ATSFontGetTable(myATSFont,
 		TRUETYPE_TAG('h','e','a','d'),
 		0, tableSize, ::CFDataGetMutableBytePtr(data),
-		&tableSize) != noErr) {
+		&tableSize) != noErr)) {
 	::CFRelease(data);
 	return;
     }
@@ -253,7 +253,7 @@ gfxMacFont::InitMetrics()
         upem = ::CGFontGetUnitsPerEm(mCGFont);
     }
 
-    if (upem < 16 || upem > 16384) {
+    if (MOZ_UNLIKELY(upem < 16 || upem > 16384)) {
         // See http://www.microsoft.com/typography/otspec/head.htm
 #ifdef DEBUG
         char warnBuf[1024];
@@ -294,7 +294,7 @@ gfxMacFont::InitMetrics()
 
        err = ::ATSFontGetHorizontalMetrics(myATSFont, kATSOptionFlagsDefault,
                                            &atsMetrics);
-       if (err != noErr) {
+       if (MOZ_UNLIKELY(err != noErr)) {
                NS_WARNING("could not get CGFontGetXHeight, no ATS fallback");
                return; // just fail -- Cameron Kludge
        }
@@ -330,7 +330,7 @@ gfxMacFont::InitMetrics()
             err = ::ATSFontGetHorizontalMetrics(myATSFont,
                                                 kATSOptionFlagsDefault,
                                                 &atsMetrics);
-            if (err != noErr) {
+            if (MOZ_UNLIKELY(err != noErr)) {
                     NS_WARNING("could not get CGFontGetXHeight again, no ATS fallback");
                     return; // just fail -- Cameron Kludge
             }
@@ -353,15 +353,15 @@ gfxMacFont::InitMetrics()
     CFDataRef cmap;
 {
     ByteCount tableSize;
-    if (::ATSFontGetTable(myATSFont, TRUETYPE_TAG('c','m','a','p'), 0, 0, NULL,
-               &tableSize) != noErr)  return;
+    if (MOZ_UNLIKELY(::ATSFontGetTable(myATSFont, TRUETYPE_TAG('c','m','a','p'), 0, 0, NULL,
+               &tableSize) != noErr))  return;
     CFMutableDataRef data = ::CFDataCreateMutable(kCFAllocatorDefault,
                                        tableSize);
-    if (!data) return;
+    if (MOZ_UNLIKELY(!data)) return;
     ::CFDataIncreaseLength(data, tableSize);
-    if (::ATSFontGetTable(myATSFont, TRUETYPE_TAG('c','m','a','p'),
+    if (MOZ_UNLIKELY(::ATSFontGetTable(myATSFont, TRUETYPE_TAG('c','m','a','p'),
                        0, tableSize,
-                       ::CFDataGetMutableBytePtr(data), &tableSize) != noErr)
+                       ::CFDataGetMutableBytePtr(data), &tableSize) != noErr))
        { ::CFRelease(data); return; }
     cmap = data;
 }
@@ -512,7 +512,7 @@ static_cast<MacOSFontEntry*>(GetFontEntry())->GetATSFontRef();
 
     err = ::ATSFontGetHorizontalMetrics(aFontRef, kATSOptionFlagsDefault,
                                         &atsMetrics);
-    if (err != noErr) {
+    if (MOZ_UNLIKELY(err != noErr)) {
 #ifdef DEBUG
         char warnBuf[1024];
         sprintf(warnBuf, "Bad font metrics for: %s err: %8.8x",
diff --git a/gfx/thebes/gfxMacPlatformFontList.mm b/gfx/thebes/gfxMacPlatformFontList.mm
index e8c46f409..3480d19ed 100644
--- a/gfx/thebes/gfxMacPlatformFontList.mm
+++ b/gfx/thebes/gfxMacPlatformFontList.mm
@@ -512,7 +512,7 @@ MacOSFontEntry::GetFontTable(uint32_t aTag)
 #endif
         OSStatus status = ::ATSFontGetTable(fontRef, aTag, 0, 0, 0,
                 &dataLength);
-        if (status != noErr) return nullptr;
+        if (MOZ_UNLIKELY(status != noErr)) return nullptr;
     }
 
     // Taking advantage of bridging CFMutableDataRef to CFDataRef.
@@ -521,9 +521,9 @@ MacOSFontEntry::GetFontTable(uint32_t aTag)
     if (!dataRef) return nullptr;
 
     ::CFDataIncreaseLength(dataRef, dataLength); // paranoia
-    if(::ATSFontGetTable(fontRef, aTag, 0, dataLength,
+    if(MOZ_UNLIKELY(::ATSFontGetTable(fontRef, aTag, 0, dataLength,
                 ::CFDataGetMutableBytePtr(dataRef),
-                &dataLength) != noErr) {
+                &dataLength) != noErr)) {
         ::CFRelease(dataRef);
         return nullptr;
     }
@@ -612,10 +612,10 @@ MacOSFontEntry::HasFontTable(uint32_t aTableTag)
 
     ByteCount sizer;
 
-    if(::ATSFontGetTableDirectory(fontRef, 0, NULL, &sizer) == noErr) {
+    if(MOZ_LIKELY(::ATSFontGetTableDirectory(fontRef, 0, NULL, &sizer) == noErr)) {
       // If the header is abnormal, try the old, slower way in case this
       // is a gap in our algorithm.
-      if (sizer <= 12 || ((sizer-12) % 16) || sizer >= 1024) {
+      if (MOZ_UNLIKELY(sizer <= 12 || ((sizer-12) % 16) || sizer >= 1024)) {
         fprintf(stderr, "Warning: TenFourFox found "
                 "abnormal font table dir in %s (%i).\n",
                  NS_ConvertUTF16toUTF8(mName).get(), sizer);
@@ -631,8 +631,8 @@ MacOSFontEntry::HasFontTable(uint32_t aTableTag)
       fprintf(stderr, "Size of %s font table directory: %i\nTables: ",
                 NS_ConvertUTF16toUTF8(mName).get(), mFontTableDir.Length());
 #endif
-      if (::ATSFontGetTableDirectory(fontRef, mFontTableDirSize,
-        reinterpret_cast<void *>(mFontTableDir.Elements()), &sizer) == noErr) {
+      if (MOZ_LIKELY(::ATSFontGetTableDirectory(fontRef, mFontTableDirSize,
+        reinterpret_cast<void *>(mFontTableDir.Elements()), &sizer) == noErr)) {
         
         // Push to platform.
     	if (!mIsDataUserFont || mIsLocalUserFont)
@@ -1175,6 +1175,7 @@ const CGFloat kTextDisplayCrossover = 20.0; // use text family below this size
 void
 gfxMacPlatformFontList::InitSystemFonts()
 {
+#ifdef __LP64__
     // system font under 10.11 are two distinct families for text/display sizes
     if (nsCocoaFeatures::OnElCapitanOrLater()) {
         mUseSizeSensitiveSystemFont = true;
@@ -1209,6 +1210,16 @@ gfxMacPlatformFontList::InitSystemFonts()
                      "system text/display font size switch point is not as expected!");
 #endif
     }
+#else   // LP64
+    // Simplified version for 10.4-10.6
+
+    NSFont* sys = [NSFont systemFontOfSize: 0.0];
+    NSString* textFamilyName = GetRealFamilyName(sys);
+    nsAutoString familyName;
+    nsCocoaUtils::GetStringForNSString(textFamilyName, familyName);
+    mSystemTextFontFamily = FindSystemFontFamily(familyName);
+    NS_ASSERTION(mSystemTextFontFamily, "null system display font family");
+#endif  // LP64
 
 #ifdef DEBUG
     // different system font API's always map to the same family under OSX, so
@@ -1583,7 +1594,7 @@ gfxMacPlatformFontList::MakePlatformFont(const nsAString& aFontName,
                                           &containerRef);
         mATSGeneration = ::ATSGetGeneration();
 
-        if (err != noErr) {
+        if (MOZ_UNLIKELY(err != noErr)) {
 #if DEBUG
             char warnBuf[1024];
             sprintf(warnBuf, "downloaded font error, ATSFontActivateFromMemory err: %d",
@@ -1596,7 +1607,7 @@ gfxMacPlatformFontList::MakePlatformFont(const nsAString& aFontName,
         // ignoring containers with multiple fonts, use the first face only for now
         err = ::ATSFontFindFromContainer(containerRef, kATSOptionFlagsDefault, 1,
                                          &fontRef, NULL);
-        if (err != noErr) {
+        if (MOZ_UNLIKELY(err != noErr)) {
 #if DEBUG
             char warnBuf[1024];
             sprintf(warnBuf, "downloaded font error, ATSFontFindFromContainer err: %d",
@@ -1611,7 +1622,7 @@ gfxMacPlatformFontList::MakePlatformFont(const nsAString& aFontName,
         OSStatus err;
         NSString *psname = NULL;
         err = ::ATSFontGetPostScriptName(fontRef, kATSOptionFlagsDefault, (CFStringRef*) (&psname));
-        if (err == noErr) {
+        if (MOZ_LIKELY(err == noErr)) {
 #if(0)
 			fprintf(stderr, "Trying: %s.\n", [psname UTF8String]);
 #endif
@@ -1679,7 +1690,7 @@ gfxMacPlatformFontList::MakePlatformFont(const nsAString& aFontName,
                              containerRef, true, false);
 
         // if succeeded and font cmap is good, return the new font
-        if (newFontEntry->mIsValid && NS_SUCCEEDED(newFontEntry->ReadCMAP())) {
+        if (MOZ_LIKELY(newFontEntry->mIsValid && NS_SUCCEEDED(newFontEntry->ReadCMAP()))) {
             return newFontEntry;
         }
 
@@ -1753,10 +1764,12 @@ gfxMacPlatformFontList::FindFamily(const nsAString& aFamily, gfxFontStyle* aStyl
 {
     // search for special system font name, -apple-system
     if (aFamily.EqualsLiteral(kSystemFont_system)) {
+#ifdef __LP64__
         if (mUseSizeSensitiveSystemFont &&
             aStyle && (aStyle->size * aDevToCssSize) >= kTextDisplayCrossover) {
             return mSystemDisplayFontFamily;
         }
+#endif
         return mSystemTextFontFamily;
     }
 
@@ -1847,11 +1860,13 @@ public:
     virtual ~MacFontInfo() {}
 
     virtual void Load() {
+#ifdef __LP64__
         nsAutoreleasePool localPool;
         // bug 975460 - async font loader crashes sometimes under 10.6, disable
         if (nsCocoaFeatures::OnLionOrLater()) {
             FontInfoData::Load();
         }
+#endif
     }
 
     // loads font data for all members of a given family