--- a/tools/profiler/core/ProfileBufferEntry.h
+++ b/tools/profiler/core/ProfileBufferEntry.h
@@ -167,21 +167,21 @@ public:
       return mAddress == aRhs.mAddress && mStreamingGen == aRhs.mStreamingGen;
     }
     bool operator!=(const JITAddress& aRhs) const { return !(*this == aRhs); }
   };
 
   struct FrameKey {
     // This cannot be a std::string, as it is not memmove compatible, which
     // is used by nsHashTable
-    nsCString mLocation;
-    mozilla::Maybe<unsigned> mLine;
-    mozilla::Maybe<unsigned> mCategory;
-    mozilla::Maybe<JITAddress> mJITAddress;
-    mozilla::Maybe<uint32_t> mJITDepth;
+    const nsCString mLocation;
+    const mozilla::Maybe<unsigned> mLine;
+    const mozilla::Maybe<unsigned> mCategory;
+    const mozilla::Maybe<JITAddress> mJITAddress;
+    const mozilla::Maybe<uint32_t> mJITDepth;
 
     explicit FrameKey(const char* aLocation)
      : mLocation(aLocation)
     {
     }
 
     FrameKey(const char* aLocation, const mozilla::Maybe<unsigned>& aLine,
              const mozilla::Maybe<unsigned>& aCategory)