--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -3843,18 +3843,18 @@ gfxFont::AddSizeOfIncludingThis(MallocSi
     aSizes->mFontInstances += aMallocSizeOf(this);
     AddSizeOfExcludingThis(aMallocSizeOf, aSizes);
 }
 
 void
 gfxFont::AddGlyphChangeObserver(GlyphChangeObserver *aObserver)
 {
     if (!mGlyphChangeObservers) {
-        mGlyphChangeObservers.reset(
-            new nsTHashtable<nsPtrHashKey<GlyphChangeObserver>>);
+        mGlyphChangeObservers =
+            MakeUnique<nsTHashtable<nsPtrHashKey<GlyphChangeObserver>>>();
     }
     mGlyphChangeObservers->PutEntry(aObserver);
 }
 
 void
 gfxFont::RemoveGlyphChangeObserver(GlyphChangeObserver *aObserver)
 {
     NS_ASSERTION(mGlyphChangeObservers, "No observers registered");