diff --git a/js/src/gc/Statistics.cpp b/js/src/gc/Statistics.cpp
index 3699f6a5ce7d..b1dadc5759aa 100644
--- a/js/src/gc/Statistics.cpp
+++ b/js/src/gc/Statistics.cpp
@@ -741,7 +741,9 @@ Statistics::formatJsonDescription(uint64_t timestamp) const
         "\"nonincremental_reason\":\"%s\","
         "\"allocated\":%u,"
         "\"added_chunks\":%d,"
-        "\"removed_chunks\":%d,";
+        "\"removed_chunks\":%d,"
+        "\"major_gc_number\":%llu,"
+        "\"minor_gc_number\":%llu,";
     char buffer[1024];
     SprintfLiteral(buffer, format,
                    (unsigned long long)timestamp,
@@ -760,7 +762,9 @@ Statistics::formatJsonDescription(uint64_t timestamp) const
                    ExplainAbortReason(nonincrementalReason_),
                    unsigned(preBytes / 1024 / 1024),
                    getCount(STAT_NEW_CHUNK),
-                   getCount(STAT_DESTROY_CHUNK));
+                   getCount(STAT_DESTROY_CHUNK),
+                   startingMajorGCNumber,
+                   startingMinorGCNumber);
     return DuplicateString(buffer);
 }
 
@@ -1072,6 +1076,7 @@ Statistics::beginGC(JSGCInvocationKind kind)
     nonincrementalReason_ = gc::AbortReason::None;
 
     preBytes = runtime->gc.usage.gcBytes();
+    startingMajorGCNumber = runtime->gc.majorGCCount();
 }
 
 void
@@ -1125,6 +1130,7 @@ void
 Statistics::beginNurseryCollection(JS::gcreason::Reason reason)
 {
     count(STAT_MINOR_GC);
+    startingMinorGCNumber = runtime->gc.minorGCCount();
     if (nurseryCollectionCallback) {
         (*nurseryCollectionCallback)(TlsContext.get(),
                                      JS::GCNurseryProgress::GC_NURSERY_COLLECTION_START,
diff --git a/js/src/gc/Statistics.h b/js/src/gc/Statistics.h
index 2a134e54f366..31c8c0ecadff 100644
--- a/js/src/gc/Statistics.h
+++ b/js/src/gc/Statistics.h
@@ -386,6 +386,10 @@ struct Statistics
     /* Allocated space before the GC started. */
     size_t preBytes;
 
+    /* GC numbers as of the beginning of the collection. */
+    uint64_t startingMinorGCNumber;
+    uint64_t startingMajorGCNumber;
+
     /* Records the maximum GC pause in an API-controlled interval (in us). */
     mutable TimeDuration maxPauseInInterval;