diff --git a/js/src/jsopcode.cpp b/js/src/jsopcode.cpp
index b5279e6ef78..218421f01ba 100644
--- a/js/src/jsopcode.cpp
+++ b/js/src/jsopcode.cpp
@@ -756,7 +756,12 @@ QuoteString(Sprinter *sp, JSString *str, uint32 quote)
                  ? Sprint(sp, "%c", (char)c) >= 0
                  : Sprint(sp, "\\%c", e[1]) >= 0;
         } else {
-            ok = Sprint(sp, (c >> 8) ? "\\u%04X" : "\\x%02X", c) >= 0;
+            /*
+             * Use \x only if the high byte is 0 and we're in a quoted string,
+             * because ECMA-262 allows only \u, not \x, in Unicode identifiers
+             * (see bug 621814).
+             */
+            ok = Sprint(sp, (qc && !(c >> 8)) ? "\\x%02X" : "\\u%04X", c) >= 0;
         }
         if (!ok)
             return NULL;
diff --git a/js/src/tests/js1_8_5/regress/jstests.list b/js/src/tests/js1_8_5/regress/jstests.list
index 5e72b82a587..03973c2e733 100644
--- a/js/src/tests/js1_8_5/regress/jstests.list
+++ b/js/src/tests/js1_8_5/regress/jstests.list
@@ -64,3 +64,4 @@ script regress-619003-1.js
 script regress-619003-2.js
 script regress-620376-1.js
 script regress-620376-2.js
+script regress-621814.js
diff --git a/js/src/tests/js1_8_5/regress/regress-621814.js b/js/src/tests/js1_8_5/regress/regress-621814.js
new file mode 100644
index 00000000000..146a2f3edad
--- /dev/null
+++ b/js/src/tests/js1_8_5/regress/regress-621814.js
@@ -0,0 +1,13 @@
+/*
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/
+ */
+var expect = 'pass';
+var actual = expect;
+function f({"\xF51F": x}) {}
+try {
+    eval(uneval(f));
+} catch (e) {
+    actual = '' + e;
+}
+reportCompare(expect, actual, "");