--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -377,16 +377,49 @@ static const char *gPrefLangNames[] = {
     "x-orya",
     "x-telu",
     "x-knda",
     "x-sinh",
     "x-tibt",
     "x-unicode",
 };
 
+// this needs to match the list of pref font.default.xx entries listed in all.js!
+// the order *must* match the order in eFontPrefLang
+static nsIAtom* gPrefLangToLangGroups[] = {
+    nsGkAtoms::x_western,
+    nsGkAtoms::Japanese,
+    nsGkAtoms::Taiwanese,
+    nsGkAtoms::Chinese,
+    nsGkAtoms::HongKongChinese,
+    nsGkAtoms::ko,
+    nsGkAtoms::x_cyrillic,
+    nsGkAtoms::el,
+    nsGkAtoms::th,
+    nsGkAtoms::he,
+    nsGkAtoms::ar,
+    nsGkAtoms::x_devanagari,
+    nsGkAtoms::x_tamil,
+    nsGkAtoms::x_armn,
+    nsGkAtoms::x_beng,
+    nsGkAtoms::x_cans,
+    nsGkAtoms::x_ethi,
+    nsGkAtoms::x_geor,
+    nsGkAtoms::x_gujr,
+    nsGkAtoms::x_guru,
+    nsGkAtoms::x_khmr,
+    nsGkAtoms::x_mlym,
+    nsGkAtoms::x_orya,
+    nsGkAtoms::x_telu,
+    nsGkAtoms::x_knda,
+    nsGkAtoms::x_sinh,
+    nsGkAtoms::x_tibt,
+    nsGkAtoms::Unicode
+};
+
 gfxPlatform::gfxPlatform()
   : mTileWidth(-1)
   , mTileHeight(-1)
   , mAzureCanvasBackendCollector(this, &gfxPlatform::GetAzureBackendInfo)
   , mApzSupportCollector(this, &gfxPlatform::GetApzSupportInfo)
 {
     mAllowDownloadableFonts = UNINITIALIZED_VALUE;
     mFallbackUsesCmaps = UNINITIALIZED_VALUE;
@@ -1469,16 +1502,29 @@ gfxPlatform::GetFontPrefLangFor(nsIAtom 
 {
     if (!aLang)
         return eFontPrefLang_Others;
     nsAutoCString lang;
     aLang->ToUTF8String(lang);
     return GetFontPrefLangFor(lang.get());
 }
 
+nsIAtom*
+gfxPlatform::GetLangGroupForPrefLang(eFontPrefLang aLang)
+{
+    // the special CJK set pref lang should be resolved into separate
+    // calls to individual CJK pref langs before getting here
+    NS_ASSERTION(aLang != eFontPrefLang_CJKSet, "unresolved CJK set pref lang");
+
+    if (uint32_t(aLang) < ArrayLength(gPrefLangToLangGroups)) {
+        return gPrefLangToLangGroups[uint32_t(aLang)];
+    }
+    return nsGkAtoms::Unicode;
+}
+
 const char*
 gfxPlatform::GetPrefLangName(eFontPrefLang aLang)
 {
     if (uint32_t(aLang) < ArrayLength(gPrefLangNames)) {
         return gPrefLangNames[uint32_t(aLang)];
     }
     return nullptr;
 }

--- a/gfx/thebes/gfxPlatform.h
+++ b/gfx/thebes/gfxPlatform.h
@@ -465,16 +465,19 @@ public:
                                   void *aClosure);
 
     // convert a lang group to enum constant (i.e. "zh-TW" ==> eFontPrefLang_ChineseTW)
     static eFontPrefLang GetFontPrefLangFor(const char* aLang);
 
     // convert a lang group atom to enum constant
     static eFontPrefLang GetFontPrefLangFor(nsIAtom *aLang);
 
+    // convert an enum constant to a lang group atom
+    static nsIAtom* GetLangGroupForPrefLang(eFontPrefLang aLang);
+
     // convert a enum constant to lang group string (i.e. eFontPrefLang_ChineseTW ==> "zh-TW")
     static const char* GetPrefLangName(eFontPrefLang aLang);
    
     // map a Unicode range (based on char code) to a font language for Preferences
     static eFontPrefLang GetFontPrefLangFor(uint8_t aUnicodeRange);
 
     // returns true if a pref lang is CJK
     static bool IsLangCJK(eFontPrefLang aLang);

--- a/gfx/thebes/gfxTextRun.cpp
+++ b/gfx/thebes/gfxTextRun.cpp
@@ -3077,17 +3077,20 @@ struct PrefFontCallbackData {
     {}
 
     nsTArray<nsRefPtr<gfxFontFamily> >& mPrefFamilies;
 
     static bool AddFontFamilyEntry(eFontPrefLang aLang, const nsAString& aName, void *aClosure)
     {
         PrefFontCallbackData *prefFontData = static_cast<PrefFontCallbackData*>(aClosure);
 
-        gfxFontFamily *family = gfxPlatformFontList::PlatformFontList()->FindFamily(aName);
+        // map pref lang to langGroup for language-sensitive lookups
+        nsIAtom* lang = gfxPlatform::GetLangGroupForPrefLang(aLang);
+        gfxFontFamily *family =
+            gfxPlatformFontList::PlatformFontList()->FindFamily(aName, lang);
         if (family) {
             prefFontData->mPrefFamilies.AppendElement(family);
         }
         return true;
     }
 };
 
 already_AddRefed<gfxFont>