--- a/tools/profiler/gecko/nsProfilerStartParams.h
+++ b/tools/profiler/gecko/nsProfilerStartParams.h
@@ -8,17 +8,20 @@
 
 #include "nsIProfiler.h"
 #include "nsString.h"
 #include "nsTArray.h"
 
 class nsProfilerStartParams : public nsIProfilerStartParams
 {
 public:
-  NS_DECL_ISUPPORTS
+  // This class can be used on multiple threads. For example, it's used for the
+  // observer notification from profiler_start, which can run on any thread but
+  // posts the notification to the main thread.
+  NS_DECL_THREADSAFE_ISUPPORTS
   NS_DECL_NSIPROFILERSTARTPARAMS
 
   nsProfilerStartParams(uint32_t aEntries,
                         double aInterval,
                         uint32_t aFeatures,
                         const nsTArray<nsCString>& aFilters);
 
 private: