--- a/tools/profiler/core/ThreadInfo.cpp
+++ b/tools/profiler/core/ThreadInfo.cpp
@@ -33,20 +33,16 @@ ThreadInfo::ThreadInfo(const char* aName
   MOZ_COUNT_CTOR(ThreadInfo);
 
   // We don't have to guess on mac
 #if defined(GP_OS_darwin)
   pthread_t self = pthread_self();
   mStackTop = pthread_get_stackaddr_np(self);
 #endif
 
-  if (aIsMainThread) {
-    mResponsiveness.emplace();
-  }
-
   // I don't know if we can assert this. But we should warn.
   MOZ_ASSERT(aThreadId >= 0, "native thread ID is < 0");
   MOZ_ASSERT(aThreadId <= INT32_MAX, "native thread ID is > INT32_MAX");
 }
 
 ThreadInfo::~ThreadInfo()
 {
   MOZ_COUNT_DTOR(ThreadInfo);
@@ -54,21 +50,25 @@ ThreadInfo::~ThreadInfo()
   delete mPseudoStack;
 }
 
 void
 ThreadInfo::StartProfiling()
 {
   mIsBeingProfiled = true;
   mPseudoStack->reinitializeOnResume();
+  if (mIsMainThread) {
+    mResponsiveness.emplace();
+  }
 }
 
 void
 ThreadInfo::StopProfiling()
 {
+  mResponsiveness.reset();
   mIsBeingProfiled = false;
 }
 
 void
 ThreadInfo::StreamJSON(ProfileBuffer* aBuffer, SpliceableJSONWriter& aWriter,
                        const TimeStamp& aStartTime, double aSinceTime)
 {
   // mUniqueStacks may already be emplaced from FlushSamplesAndMarkers.

--- a/tools/profiler/core/ThreadInfo.h
+++ b/tools/profiler/core/ThreadInfo.h
@@ -62,21 +62,22 @@ public:
   void StreamJSON(ProfileBuffer* aBuffer, SpliceableJSONWriter& aWriter,
                   const mozilla::TimeStamp& aStartTime, double aSinceTime);
 
   // Call this method when the JS entries inside the buffer are about to
   // become invalid, i.e., just before JS shutdown.
   void FlushSamplesAndMarkers(ProfileBuffer* aBuffer,
                               const mozilla::TimeStamp& aStartTime);
 
-  // Returns nullptr if this is not the main thread.
+  // Returns nullptr if this is not the main thread or if this thread is not
+  // being profiled.
   ThreadResponsiveness* GetThreadResponsiveness()
   {
     ThreadResponsiveness* responsiveness = mResponsiveness.ptrOr(nullptr);
-    MOZ_ASSERT(!!responsiveness == mIsMainThread);
+    MOZ_ASSERT(!!responsiveness == (mIsMainThread && mIsBeingProfiled));
     return responsiveness;
   }
 
 private:
   bool mIsBeingProfiled;
 
   // JS frames in the buffer may require a live JSRuntime to stream (e.g.,
   // stringifying JIT frames). In the case of JSRuntime destruction,