diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 54509dae62f2..22f409fd8c77 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -514,6 +514,8 @@ Parser<ParseHandler>::Parser(ExclusiveContext *cx, LifoAlloc *alloc,
     sawDeprecatedDestructuringForIn(false),
     sawDeprecatedLegacyGenerator(false),
     sawDeprecatedExpressionClosure(false),
+    sawDeprecatedLetBlock(false),
+    sawDeprecatedLetExpression(false),
     handler(cx, *alloc, tokenStream, foldConstants, syntaxParser, lazyOuterFunction)
 {
     {
@@ -3657,11 +3659,13 @@ Parser<ParseHandler>::letBlock(LetContext letContext)
         if (!expr)
             return null();
         MUST_MATCH_TOKEN(TOK_RC, JSMSG_CURLY_AFTER_LET);
+        sawDeprecatedLetBlock = true;
     } else {
         MOZ_ASSERT(letContext == LetExpression);
         expr = assignExpr();
         if (!expr)
             return null();
+        sawDeprecatedLetExpression = true;
     }
     handler.setLexicalScopeBody(block, expr);
     PopStatementPC(tokenStream, pc);
@@ -8334,6 +8338,8 @@ Parser<ParseHandler>::accumulateTelemetry()
         DeprecatedDestructuringForIn = 1, // JS 1.7 only
         DeprecatedLegacyGenerator = 2,    // JS 1.7+
         DeprecatedExpressionClosure = 3,  // Added in JS 1.8, but not version-gated
+        DeprecatedLetBlock = 4,           // Added in JS 1.7, but not version-gated
+        DeprecatedLetExpression = 5,      // Added in JS 1.7, but not version-gated
     };
 
     // Hazard analysis can't tell that the telemetry callbacks don't GC.
@@ -8348,6 +8354,10 @@ Parser<ParseHandler>::accumulateTelemetry()
         (*cb)(JS_TELEMETRY_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT, DeprecatedLegacyGenerator);
     if (sawDeprecatedExpressionClosure)
         (*cb)(JS_TELEMETRY_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT, DeprecatedExpressionClosure);
+    if (sawDeprecatedLetBlock)
+        (*cb)(JS_TELEMETRY_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT, DeprecatedLetBlock);
+    if (sawDeprecatedLetExpression)
+        (*cb)(JS_TELEMETRY_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT, DeprecatedLetExpression);
 }
 
 template class Parser<FullParseHandler>;
diff --git a/js/src/frontend/Parser.h b/js/src/frontend/Parser.h
index 519916dae54b..18acc4b910b9 100644
--- a/js/src/frontend/Parser.h
+++ b/js/src/frontend/Parser.h
@@ -377,6 +377,8 @@ class Parser : private JS::AutoGCRooter, public StrictModeGetter
     bool sawDeprecatedDestructuringForIn:1;
     bool sawDeprecatedLegacyGenerator:1;
     bool sawDeprecatedExpressionClosure:1;
+    bool sawDeprecatedLetBlock:1;
+    bool sawDeprecatedLetExpression:1;
 
     typedef typename ParseHandler::Node Node;
     typedef typename ParseHandler::DefinitionNode DefinitionNode;
diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index 9554aa2ee05b..e85a181e1be2 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -3012,7 +3012,7 @@ AccumulateTelemetryCallback(int id, uint32_t sample)
         Telemetry::Accumulate(Telemetry::GC_SCC_SWEEP_MAX_PAUSE_MS, sample);
         break;
       case JS_TELEMETRY_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT:
-        MOZ_ASSERT(sample <= 3);
+        MOZ_ASSERT(sample <= 5);
         Telemetry::Accumulate(Telemetry::JS_DEPRECATED_LANGUAGE_EXTENSIONS_IN_CONTENT, sample);
         break;
       default:
diff --git a/toolkit/components/telemetry/Histograms.json b/toolkit/components/telemetry/Histograms.json
index 9f049e28a175..af51df0e199a 100644
--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -322,7 +322,7 @@
     "expires_in_version": "never",
     "kind": "enumerated",
     "n_values": 10,
-    "description": "Use of SpiderMonkey's deprecated language extensions in web content: ForEach, DestructuringForIn, LegacyGenerator, ExpressionClosure"
+    "description": "Use of SpiderMonkey's deprecated language extensions in web content: ForEach=0, DestructuringForIn=1, LegacyGenerator=2, ExpressionClosure=3, LetBlock=4, LetExpression=5"
   },
   "TELEMETRY_PING": {
     "expires_in_version": "default",