diff --git a/gfx/thebes/CoreTextFontList.cpp b/gfx/thebes/CoreTextFontList.cpp
index 196f90ef01d..459abdb76f3 100644
--- a/gfx/thebes/CoreTextFontList.cpp
+++ b/gfx/thebes/CoreTextFontList.cpp
@@ -374,12 +374,17 @@ nsresult CTFontEntry::ReadCMAP(FontInfoData* aFontInfoData) {
 }
 
 gfxFont* CTFontEntry::CreateFontInstance(const gfxFontStyle* aFontStyle) {
-  RefPtr<UnscaledFontMac> unscaledFont(mUnscaledFont);
+  RefPtr<UnscaledFontMac> unscaledFont;
+  {
+    AutoReadLock lock(mLock);
+    unscaledFont = RefPtr<UnscaledFontMac>(mUnscaledFont);
+  }
   if (!unscaledFont) {
     CGFontRef baseFont = GetFontRef();
     if (!baseFont) {
       return nullptr;
     }
+    AutoWriteLock lock(mLock);
     unscaledFont = new UnscaledFontMac(baseFont, mIsDataUserFont);
     mUnscaledFont = unscaledFont;
   }
diff --git a/gfx/thebes/gfxDWriteFontList.cpp b/gfx/thebes/gfxDWriteFontList.cpp
index e303b149635..6a3cbfd64ee 100644
--- a/gfx/thebes/gfxDWriteFontList.cpp
+++ b/gfx/thebes/gfxDWriteFontList.cpp
@@ -706,7 +706,11 @@ gfxFont* gfxDWriteFontEntry::CreateFontInstance(
       useBoldSim ? DWRITE_FONT_SIMULATIONS_BOLD : DWRITE_FONT_SIMULATIONS_NONE;
   ThreadSafeWeakPtr<UnscaledFontDWrite>& unscaledFontPtr =
       useBoldSim ? mUnscaledFontBold : mUnscaledFont;
-  RefPtr<UnscaledFontDWrite> unscaledFont(unscaledFontPtr);
+  RefPtr<UnscaledFontDWrite> unscaledFont;
+  {
+    AutoReadLock lock(mLock);
+    unscaledFont = RefPtr<UnscaledFontDWrite>((unscaledFontPtr);
+  }
   if (!unscaledFont) {
     RefPtr<IDWriteFontFace> fontFace;
     nsresult rv =
@@ -714,6 +718,7 @@ gfxFont* gfxDWriteFontEntry::CreateFontInstance(
     if (NS_FAILED(rv)) {
       return nullptr;
     }
+    AutoWriteLock lock(mLock);
     // Only pass in the underlying IDWriteFont if the unscaled font doesn't
     // reflect a data font. This signals whether or not we can safely query
     // a descriptor to represent the font for various transport use-cases.
diff --git a/gfx/thebes/gfxFT2FontList.cpp b/gfx/thebes/gfxFT2FontList.cpp
index ff0f22222be..bdbfa7f2cb5 100644
--- a/gfx/thebes/gfxFT2FontList.cpp
+++ b/gfx/thebes/gfxFT2FontList.cpp
@@ -226,8 +226,13 @@ gfxFont* FT2FontEntry::CreateFontInstance(const gfxFontStyle* aStyle) {
     loadFlags &= ~FT_LOAD_NO_AUTOHINT;
   }
 
-  RefPtr<UnscaledFontFreeType> unscaledFont(mUnscaledFont);
+  RefPtr<UnscaledFontFreeType> unscaledFont;
+  {
+    AutoReadLock lock(mLock);
+    unscaledFont = RefPtr<UnscaledFontFreeType>(mUnscaledFont);
+  }
   if (!unscaledFont) {
+    AutoWriteLock lock(mLock);
     RefPtr<SharedFTFace> origFace(mFTFace);
     unscaledFont = !mFilename.IsEmpty() && mFilename[0] == '/'
                        ? new UnscaledFontFreeType(mFilename.get(), mFTFontIndex,