--- a/js/src/jsscan.cpp
+++ b/js/src/jsscan.cpp
@@ -1050,20 +1050,33 @@ TokenStream::getTokenInternal()
             } else if (kw->tokentype == TOK_STRICT_RESERVED) {
                 if (isStrictMode()
                     ? !ReportStrictModeError(cx, this, NULL, NULL, JSMSG_RESERVED_ID, kw->chars)
                     : !ReportCompileErrorNumber(cx, this, NULL,
                                                 JSREPORT_STRICT | JSREPORT_WARNING,
                                                 JSMSG_RESERVED_ID, kw->chars)) {
                     goto error;
                 }
-            } else if (kw->version <= VersionNumber(version)) {
-                tt = kw->tokentype;
-                tp->t_op = (JSOp) kw->op;
-                goto out;
+            } else {
+                if (kw->version <= VersionNumber(version)) {
+                    tt = kw->tokentype;
+                    tp->t_op = (JSOp) kw->op;
+                    goto out;
+                }
+
+                /*
+                 * let/yield are a Mozilla extension starting in JS1.7. If we
+                 * aren't parsing for a version supporting these extensions,
+                 * conform to ES5 and forbid these names in strict mode.
+                 */
+                if ((kw->tokentype == TOK_LET || kw->tokentype == TOK_YIELD) &&
+                    !ReportStrictModeError(cx, this, NULL, NULL, JSMSG_RESERVED_ID, kw->chars))
+                {
+                    goto error;
+                }
             }
         }
 
         atom = atomize(cx, tokenbuf);
         if (!atom)
             goto error;
         tp->t_op = JSOP_NAME;
         tp->t_atom = atom;

--- a/js/src/tests/ecma_5/misc/future-reserved-words.js
+++ b/js/src/tests/ecma_5/misc/future-reserved-words.js
@@ -9,35 +9,37 @@ var summary = "Implement FutureReservedW
 
 print(BUGNUMBER + ": " + summary);
 
 /**************
  * BEGIN TEST *
  **************/
 
 var futureReservedWords =
-  ["class",
+  [
+   "class",
    // "const", // Mozilla extension enabled even for versionless code
    "enum",
    "export",
    "extends",
    "import",
-   "super"];
+   "super",
+  ];
 
 var strictFutureReservedWords =
   [
    "implements",
    "interface",
-   // "let", // Mozilla extension, strict checks disabled for now
+   "let", // enabled: this file doesn't execute as JS1.7
    "package",
    "private",
    "protected",
    "public",
    "static",
-   // "yield", // Mozilla extension, strict checks disabled for now
+   "yield", // enabled: this file doesn't execute as JS1.7
   ];
 
 function testWord(word, expectNormal, expectStrict)
 {
   var actual, status;
 
   // USE IN VARIABLE DECLARATION