diff --git a/mozglue/baseprofiler/core/ProfileBufferEntry.cpp b/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
index 4915b400ded0..9894ee6c9235 100644
--- a/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
+++ b/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
@@ -326,7 +326,8 @@ void UniqueStacks::StreamNonJITFrame(const FrameKey& aFrame) {
     OPTIMIZATIONS = 3,
     LINE = 4,
     COLUMN = 5,
-    CATEGORY = 6
+    CATEGORY = 6,
+    SUBCATEGORY = 7
   };
 
   AutoArraySchemaWriter writer(mFrameTableWriter, *mUniqueStrings);
@@ -344,6 +345,7 @@ void UniqueStacks::StreamNonJITFrame(const FrameKey& aFrame) {
     const ProfilingCategoryPairInfo& info =
         GetProfilingCategoryPairInfo(*data.mCategoryPair);
     writer.IntElement(CATEGORY, uint32_t(info.mCategory));
+    writer.IntElement(SUBCATEGORY, info.mSubcategoryIndex);
   }
 }
 
diff --git a/mozglue/baseprofiler/core/ProfileBufferEntry.h b/mozglue/baseprofiler/core/ProfileBufferEntry.h
index 78cf8384c47e..4f63c65caf9c 100644
--- a/mozglue/baseprofiler/core/ProfileBufferEntry.h
+++ b/mozglue/baseprofiler/core/ProfileBufferEntry.h
@@ -344,11 +344,14 @@ class UniqueStacks {
 //     "schema":
 //     {
 //       "location": 0,       /* index into stringTable */
-//       "implementation": 1, /* index into stringTable */
-//       "optimizations": 2,  /* arbitrary JSON */
-//       "line": 3,           /* number */
-//       "column": 4,         /* number */
-//       "category": 5        /* number */
+//       "relevantForJS": 1,  /* bool */
+//       "implementation": 2, /* index into stringTable */
+//       "optimizations": 3,  /* arbitrary JSON */
+//       "line": 4,           /* number */
+//       "column": 5,         /* number */
+//       "category": 6        /* index into profile.meta.categories */
+//       "subcategory": 7     /* index into
+//       profile.meta.categories[category].subcategories */
 //     },
 //     "data":
 //     [
diff --git a/mozglue/baseprofiler/core/ProfiledThreadData.cpp b/mozglue/baseprofiler/core/ProfiledThreadData.cpp
index 505350e3914b..f4ed9a784ba5 100644
--- a/mozglue/baseprofiler/core/ProfiledThreadData.cpp
+++ b/mozglue/baseprofiler/core/ProfiledThreadData.cpp
@@ -64,6 +64,7 @@ void ProfiledThreadData::StreamJSON(const ProfileBuffer& aBuffer,
         schema.WriteField("line");
         schema.WriteField("column");
         schema.WriteField("category");
+        schema.WriteField("subcategory");
       }
 
       aWriter.StartArrayProperty("data");
diff --git a/mozglue/baseprofiler/core/platform.cpp b/mozglue/baseprofiler/core/platform.cpp
index a50e7a1bf197..fe2ec1a74db7 100644
--- a/mozglue/baseprofiler/core/platform.cpp
+++ b/mozglue/baseprofiler/core/platform.cpp
@@ -1497,14 +1497,35 @@ void AppendSharedLibraries(JSONWriter& aWriter) {
 }
 
 static void StreamCategories(SpliceableJSONWriter& aWriter) {
-  // Same order as ProfilingCategory.
+  // Same order as ProfilingCategory. Format:
+  // [
+  //   {
+  //     name: "Idle",
+  //     color: "transparent",
+  //     subcategories: ["Other"],
+  //   },
+  //   {
+  //     name: "Other",
+  //     color: "grey",
+  //     subcategories: [
+  //       "JSM loading",
+  //       "Subprocess launching",
+  //       "DLL loading"
+  //     ]
+  //   },
+  //   ...
+  // ]
 
 #  define CATEGORY_JSON_BEGIN_CATEGORY(name, labelAsString, color) \
     aWriter.Start();                                               \
     aWriter.StringProperty("name", labelAsString);                 \
-    aWriter.StringProperty("color", color);
-#  define CATEGORY_JSON_SUBCATEGORY(category, name, labelAsString)
-#  define CATEGORY_JSON_END_CATEGORY aWriter.EndObject();
+    aWriter.StringProperty("color", color);                        \
+    aWriter.StartArrayProperty("subcategories");
+#  define CATEGORY_JSON_SUBCATEGORY(supercategory, name, labelAsString) \
+    aWriter.StringElement(labelAsString);
+#  define CATEGORY_JSON_END_CATEGORY \
+    aWriter.EndArray();              \
+    aWriter.EndObject();
 
   BASE_PROFILING_CATEGORY_LIST(CATEGORY_JSON_BEGIN_CATEGORY,
                                CATEGORY_JSON_SUBCATEGORY,
@@ -1522,7 +1543,7 @@ static void StreamMetaJSCustomObject(PSLockRef aLock,
                                      bool aIsShuttingDown) {
   MOZ_RELEASE_ASSERT(CorePS::Exists() && ActivePS::Exists(aLock));
 
-  aWriter.IntProperty("version", 15);
+  aWriter.IntProperty("version", 16);
 
   // The "startTime" field holds the number of milliseconds since midnight
   // January 1, 1970 GMT. This grotty code computes (Now - (Now -
diff --git a/tools/profiler/core/ProfileBufferEntry.cpp b/tools/profiler/core/ProfileBufferEntry.cpp
index e5ac69880601..c370e47902db 100644
--- a/tools/profiler/core/ProfileBufferEntry.cpp
+++ b/tools/profiler/core/ProfileBufferEntry.cpp
@@ -484,7 +484,8 @@ void UniqueStacks::StreamNonJITFrame(const FrameKey& aFrame) {
     OPTIMIZATIONS = 3,
     LINE = 4,
     COLUMN = 5,
-    CATEGORY = 6
+    CATEGORY = 6,
+    SUBCATEGORY = 7
   };
 
   AutoArraySchemaWriter writer(mFrameTableWriter, *mUniqueStrings);
@@ -502,6 +503,7 @@ void UniqueStacks::StreamNonJITFrame(const FrameKey& aFrame) {
     const JS::ProfilingCategoryPairInfo& info =
         JS::GetProfilingCategoryPairInfo(*data.mCategoryPair);
     writer.IntElement(CATEGORY, uint32_t(info.mCategory));
+    writer.IntElement(SUBCATEGORY, info.mSubcategoryIndex);
   }
 }
 
@@ -603,7 +605,8 @@ static void StreamJITFrame(JSContext* aContext, SpliceableJSONWriter& aWriter,
     OPTIMIZATIONS = 3,
     LINE = 4,
     COLUMN = 5,
-    CATEGORY = 6
+    CATEGORY = 6,
+    SUBCATEGORY = 7
   };
 
   AutoArraySchemaWriter writer(aWriter, aUniqueStrings);
@@ -626,6 +629,11 @@ static void StreamJITFrame(JSContext* aContext, SpliceableJSONWriter& aWriter,
                                       aJITFrame);
         });
   }
+
+  const JS::ProfilingCategoryPairInfo& info =
+      JS::GetProfilingCategoryPairInfo(JS::ProfilingCategoryPair::JS);
+  writer.IntElement(CATEGORY, uint32_t(info.mCategory));
+  writer.IntElement(SUBCATEGORY, info.mSubcategoryIndex);
 }
 
 struct CStringWriteFunc : public JSONWriteFunc {
diff --git a/tools/profiler/core/ProfileBufferEntry.h b/tools/profiler/core/ProfileBufferEntry.h
index d63a77c2f5a5..a1757b45e6c9 100644
--- a/tools/profiler/core/ProfileBufferEntry.h
+++ b/tools/profiler/core/ProfileBufferEntry.h
@@ -469,11 +469,14 @@ class UniqueStacks {
 //     "schema":
 //     {
 //       "location": 0,       /* index into stringTable */
-//       "implementation": 1, /* index into stringTable */
-//       "optimizations": 2,  /* arbitrary JSON */
-//       "line": 3,           /* number */
-//       "column": 4,         /* number */
-//       "category": 5        /* number */
+//       "relevantForJS": 1,  /* bool */
+//       "implementation": 2, /* index into stringTable */
+//       "optimizations": 3,  /* arbitrary JSON */
+//       "line": 4,           /* number */
+//       "column": 5,         /* number */
+//       "category": 6        /* index into profile.meta.categories */
+//       "subcategory": 7     /* index into
+//       profile.meta.categories[category].subcategories */
 //     },
 //     "data":
 //     [
diff --git a/tools/profiler/core/ProfiledThreadData.cpp b/tools/profiler/core/ProfiledThreadData.cpp
index c140a99db596..1f3be57dbc6f 100644
--- a/tools/profiler/core/ProfiledThreadData.cpp
+++ b/tools/profiler/core/ProfiledThreadData.cpp
@@ -87,6 +87,7 @@ void ProfiledThreadData::StreamJSON(const ProfileBuffer& aBuffer,
         schema.WriteField("line");
         schema.WriteField("column");
         schema.WriteField("category");
+        schema.WriteField("subcategory");
       }
 
       aWriter.StartArrayProperty("data");
diff --git a/tools/profiler/core/platform.cpp b/tools/profiler/core/platform.cpp
index d5bf859cae73..3d85120814fb 100644
--- a/tools/profiler/core/platform.cpp
+++ b/tools/profiler/core/platform.cpp
@@ -1842,14 +1842,35 @@ static void StreamTaskTracer(PSLockRef aLock, SpliceableJSONWriter& aWriter) {
 }
 
 static void StreamCategories(SpliceableJSONWriter& aWriter) {
-  // Same order as ProfilingCategory.
+  // Same order as ProfilingCategory. Format:
+  // [
+  //   {
+  //     name: "Idle",
+  //     color: "transparent",
+  //     subcategories: ["Other"],
+  //   },
+  //   {
+  //     name: "Other",
+  //     color: "grey",
+  //     subcategories: [
+  //       "JSM loading",
+  //       "Subprocess launching",
+  //       "DLL loading"
+  //     ]
+  //   },
+  //   ...
+  // ]
 
 #define CATEGORY_JSON_BEGIN_CATEGORY(name, labelAsString, color) \
   aWriter.Start();                                               \
   aWriter.StringProperty("name", labelAsString);                 \
-  aWriter.StringProperty("color", color);
-#define CATEGORY_JSON_SUBCATEGORY(category, name, labelAsString)
-#define CATEGORY_JSON_END_CATEGORY aWriter.EndObject();
+  aWriter.StringProperty("color", color);                        \
+  aWriter.StartArrayProperty("subcategories");
+#define CATEGORY_JSON_SUBCATEGORY(supercategory, name, labelAsString) \
+  aWriter.StringElement(labelAsString);
+#define CATEGORY_JSON_END_CATEGORY \
+  aWriter.EndArray();              \
+  aWriter.EndObject();
 
   PROFILING_CATEGORY_LIST(CATEGORY_JSON_BEGIN_CATEGORY,
                           CATEGORY_JSON_SUBCATEGORY, CATEGORY_JSON_END_CATEGORY)
@@ -1864,7 +1885,7 @@ static void StreamMetaJSCustomObject(PSLockRef aLock,
                                      bool aIsShuttingDown) {
   MOZ_RELEASE_ASSERT(CorePS::Exists() && ActivePS::Exists(aLock));
 
-  aWriter.IntProperty("version", 15);
+  aWriter.IntProperty("version", 16);
 
   // The "startTime" field holds the number of milliseconds since midnight
   // January 1, 1970 GMT. This grotty code computes (Now - (Now -