diff --git a/gfx/thebes/gfxMacFont.cpp b/gfx/thebes/gfxMacFont.cpp
index 1a614bda912..7a9936d2694 100644
--- a/gfx/thebes/gfxMacFont.cpp
+++ b/gfx/thebes/gfxMacFont.cpp
@@ -268,6 +268,16 @@ gfxMacFont::InitMetrics()
     mAdjustedSize = PR_MAX(mStyle.size, 1.0f);
     mFUnitsConvFactor = mAdjustedSize / upem;
 
+    // For CFF fonts, when scaling values read from CGFont* APIs, we need to
+    // use CG's idea of unitsPerEm, which may differ from the "true" value in
+    // the head table of the font (see bug 580863)
+    gfxFloat cgConvFactor;
+    if (static_cast<MacOSFontEntry*>(mFontEntry.get())->IsCFF()) {
+        cgConvFactor = mAdjustedSize / ::CGFontGetUnitsPerEm(mCGFont);
+    } else {
+        cgConvFactor = mFUnitsConvFactor;
+    }
+
     // Try to read 'sfnt' metrics; for local, non-sfnt fonts ONLY, fall back to
     // platform APIs. The InitMetrics...() functions will set mIsValid on success.
     if (!InitMetricsFromSfntTables(mMetrics) &&
@@ -279,7 +289,7 @@ gfxMacFont::InitMetrics()
     }
 
     if (mMetrics.xHeight == 0.0) {
-        mMetrics.xHeight = ::CGFontGetXHeight(mCGFont) * mFUnitsConvFactor;
+        mMetrics.xHeight = ::CGFontGetXHeight(mCGFont) * cgConvFactor;
     }
 
     if (mStyle.sizeAdjust != 0.0 && mStyle.size > 0.0 &&
@@ -288,6 +298,11 @@ gfxMacFont::InitMetrics()
         gfxFloat aspect = mMetrics.xHeight / mStyle.size;
         mAdjustedSize = mStyle.GetAdjustedSize(aspect);
         mFUnitsConvFactor = mAdjustedSize / upem;
+        if (static_cast<MacOSFontEntry*>(mFontEntry.get())->IsCFF()) {
+            cgConvFactor = mAdjustedSize / ::CGFontGetUnitsPerEm(mCGFont);
+        } else {
+            cgConvFactor = mFUnitsConvFactor;
+        }
         mMetrics.xHeight = 0.0;
         if (!InitMetricsFromSfntTables(mMetrics) &&
             (!mFontEntry->IsUserFont() || mFontEntry->IsLocalUserFont())) {
@@ -299,7 +314,7 @@ gfxMacFont::InitMetrics()
             return;
         }
         if (mMetrics.xHeight == 0.0) {
-            mMetrics.xHeight = ::CGFontGetXHeight(mCGFont) * mFUnitsConvFactor;
+            mMetrics.xHeight = ::CGFontGetXHeight(mCGFont) * cgConvFactor;
         }
     }
 
@@ -317,7 +332,8 @@ gfxMacFont::InitMetrics()
 
     PRUint32 glyphID;
     if (mMetrics.aveCharWidth <= 0) {
-        mMetrics.aveCharWidth = GetCharWidth(cmap, 'x', &glyphID);
+        mMetrics.aveCharWidth = GetCharWidth(cmap, 'x', &glyphID,
+                                             cgConvFactor);
         if (glyphID == 0) {
             // we didn't find 'x', so use maxAdvance rather than zero
             mMetrics.aveCharWidth = mMetrics.maxAdvance;
@@ -326,14 +342,15 @@ gfxMacFont::InitMetrics()
     mMetrics.aveCharWidth += mSyntheticBoldOffset;
     mMetrics.maxAdvance += mSyntheticBoldOffset;
 
-    mMetrics.spaceWidth = GetCharWidth(cmap, ' ', &glyphID);
+    mMetrics.spaceWidth = GetCharWidth(cmap, ' ', &glyphID, cgConvFactor);
     if (glyphID == 0) {
         // no space glyph?!
         mMetrics.spaceWidth = mMetrics.aveCharWidth;
     }
     mSpaceGlyph = glyphID;
 
-    mMetrics.zeroOrAveCharWidth = GetCharWidth(cmap, '0', &glyphID);
+    mMetrics.zeroOrAveCharWidth = GetCharWidth(cmap, '0', &glyphID,
+                                               cgConvFactor);
     if (glyphID == 0) {
         mMetrics.zeroOrAveCharWidth = mMetrics.aveCharWidth;
     }
@@ -360,7 +377,7 @@ gfxMacFont::InitMetrics()
 
 gfxFloat
 gfxMacFont::GetCharWidth(CFDataRef aCmap, PRUnichar aUniChar,
-                         PRUint32 *aGlyphID)
+                         PRUint32 *aGlyphID, gfxFloat aConvFactor)
 {
     CGGlyph glyph = 0;
     
@@ -377,7 +394,7 @@ gfxMacFont::GetCharWidth(CFDataRef aCmap, PRUnichar aUniChar,
     if (glyph) {
         int advance;
         if (::CGFontGetGlyphAdvances(mCGFont, &glyph, 1, &advance)) {
-            return advance * mFUnitsConvFactor;
+            return advance * aConvFactor;
         }
     }
 
diff --git a/gfx/thebes/gfxMacFont.h b/gfx/thebes/gfxMacFont.h
index cf9703650d5..b1320c980f1 100644
--- a/gfx/thebes/gfxMacFont.h
+++ b/gfx/thebes/gfxMacFont.h
@@ -85,10 +85,10 @@ protected:
     void InitMetrics();
     void InitMetricsFromATSMetrics();
 
-    // Get width and glyph ID for a character; requires that
-    // mFUnitsConvFactor has been set before this is called
+    // Get width and glyph ID for a character; uses aConvFactor
+    // to convert font units as returned by CG to actual dimensions
     gfxFloat GetCharWidth(CFDataRef aCmap, PRUnichar aUniChar,
-                          PRUint32 *aGlyphID);
+                          PRUint32 *aGlyphID, gfxFloat aConvFactor);
 
     static void DestroyBlobFunc(void* aUserData);
 
diff --git a/gfx/thebes/gfxMacPlatformFontList.h b/gfx/thebes/gfxMacPlatformFontList.h
index b16148847fa..51f91581750 100644
--- a/gfx/thebes/gfxMacPlatformFontList.h
+++ b/gfx/thebes/gfxMacPlatformFontList.h
@@ -70,6 +70,8 @@ public:
 
     virtual nsresult GetFontTable(PRUint32 aTableTag, nsTArray<PRUint8>& aBuffer);
 
+    PRBool IsCFF();
+
 protected:
     // for use with data fonts
     MacOSFontEntry(const nsAString& aPostscriptName, ATSFontRef aFontRef,
@@ -81,6 +83,8 @@ protected:
     ATSFontRef mATSFontRef;
     PRPackedBool mATSFontRefInitialized;
     PRPackedBool mRequiresAAT;
+    PRPackedBool mIsCFF;
+    PRPackedBool mIsCFFInitialized;
 };
 
 class gfxMacPlatformFontList : public gfxPlatformFontList {
diff --git a/gfx/thebes/gfxMacPlatformFontList.mm b/gfx/thebes/gfxMacPlatformFontList.mm
index 0d6609ee792..dc1800143a6 100644
--- a/gfx/thebes/gfxMacPlatformFontList.mm
+++ b/gfx/thebes/gfxMacPlatformFontList.mm
@@ -137,7 +137,9 @@ MacOSFontEntry::MacOSFontEntry(const nsAString& aPostscriptName,
     : gfxFontEntry(aPostscriptName, aFamily, aIsStandardFace),
       mATSFontRef(0),
       mATSFontRefInitialized(PR_FALSE),
-      mRequiresAAT(PR_FALSE)
+      mRequiresAAT(PR_FALSE),
+      mIsCFF(PR_FALSE),
+      mIsCFFInitialized(PR_FALSE)
 {
     mWeight = aWeight;
 }
@@ -148,7 +150,8 @@ MacOSFontEntry::MacOSFontEntry(const nsAString& aPostscriptName, ATSFontRef aFon
     : gfxFontEntry(aPostscriptName),
       mATSFontRef(aFontRef),
       mATSFontRefInitialized(PR_TRUE),
-      mRequiresAAT(PR_FALSE)
+      mRequiresAAT(PR_FALSE),
+      mIsCFFInitialized(PR_FALSE)
 {
     // xxx - stretch is basically ignored for now
 
@@ -313,6 +316,26 @@ MacOSFontEntry::CreateFontInstance(const gfxFontStyle *aFontStyle, PRBool aNeeds
     return new gfxMacFont(this, aFontStyle, aNeedsBold);
 }
 
+PRBool
+MacOSFontEntry::IsCFF()
+{
+    if (!mIsCFFInitialized) {
+        mIsCFFInitialized = PR_TRUE;
+        ATSFontRef fontRef = GetFontRef();
+        if (fontRef != (ATSFontRef)kATSUInvalidFontID) {
+            ByteCount dataLength;
+            OSStatus status = ::ATSFontGetTable(fontRef,
+                                                TRUETYPE_TAG('C','F','F',' '),
+                                                0, 0, 0, &dataLength);
+            if (status == noErr && dataLength > 0) {
+                mIsCFF = PR_TRUE;
+            }
+        }
+    }
+
+    return mIsCFF;
+}
+
 
 /* gfxMacFontFamily */
 #pragma mark-