diff --git a/gfx/thebes/gfxFont.cpp b/gfx/thebes/gfxFont.cpp
index 383cc19042..9c3cf6a3d1 100644
--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -860,6 +860,10 @@ gfxFont::gfxFont(gfxFontEntry *aFontEntry, const gfxFontStyle *aFontStyle,
     ++gFontCount;
 #endif
     mKerningSet = HasFeatureSet(HB_TAG('k','e','r','n'), mKerningEnabled);
+
+    // Ensure the gfxFontEntry's unitsPerEm and extents fields are initialized,
+    // so that GetFontExtents can use them without risk of races.
+    Unused << mFontEntry->UnitsPerEm();
 }
 
 gfxFont::~gfxFont()