diff --git a/js/src/builtin/Object.cpp b/js/src/builtin/Object.cpp
index f27093684..647de2c20 100644
--- a/js/src/builtin/Object.cpp
+++ b/js/src/builtin/Object.cpp
@@ -1176,7 +1176,7 @@ static const JSFunctionSpec object_methods[] = {
     JS_FN(js_toString_str,             obj_toString,                0,0),
     JS_SELF_HOSTED_FN(js_toLocaleString_str, "Object_toLocaleString", 0,JSPROP_DEFINE_LATE),
     JS_FN(js_valueOf_str,              obj_valueOf,                 0,0),
-#if (0) // JS_HAS_OBJ_WATCHPOINT // TenFourFox issue 575
+#if (0) // JS_HAS_OBJ_WATCHPOINT // TenFourFox issue 575 // flip this for confidence testing
     JS_FN(js_watch_str,                obj_watch,                   2,0),
     JS_FN(js_unwatch_str,              obj_unwatch,                 1,0),
 #endif
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index bf94da4af..faea25b06 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -1258,7 +1258,11 @@ CheckSetConstOp(JSOp op, ParseNode* pn)
         switch (op) {
           case JSOP_GETLOCAL: case JSOP_GETALIASEDVAR: break;
           case JSOP_INITLEXICAL: case JSOP_INITALIASEDLEXICAL: break;
-          case JSOP_SETLOCAL: break; /* return JSOP_THROWSETCONST; */
+#if (0) // flip this for confidence testing
+          case JSOP_SETLOCAL: return JSOP_THROWSETCONST;
+#else
+          case JSOP_SETLOCAL: break;
+#endif
           case JSOP_SETALIASEDVAR: return JSOP_THROWSETALIASEDCONST;
           default: MOZ_CRASH("unexpected set var op");
         }
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index df0a2c7cd..79f793eb2 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -4735,8 +4735,6 @@ JS_NewStringCopyN(JSContext* cx, const char* s, size_t n)
 {
     AssertHeapIsIdle(cx);
     CHECK_REQUEST(cx);
-    if (!n)
-        return cx->names().empty;
     return NewStringCopyN<CanGC>(cx, s, n);
 }
 
@@ -4745,7 +4743,7 @@ JS_NewStringCopyZ(JSContext* cx, const char* s)
 {
     AssertHeapIsIdle(cx);
     CHECK_REQUEST(cx);
-    if (!s || !*s)
+    if (!s)
         return cx->runtime()->emptyString;
     return NewStringCopyZ<CanGC>(cx, s);
 }
diff --git a/js/src/vm/String.cpp b/js/src/vm/String.cpp
index 37ad492d4..364d6a0d7 100644
--- a/js/src/vm/String.cpp
+++ b/js/src/vm/String.cpp
@@ -790,9 +790,11 @@ StaticStrings::init(JSContext* cx)
     static_assert(UNIT_STATIC_LIMIT - 1 <= JSString::MAX_LATIN1_CHAR,
                   "Unit strings must fit in Latin1Char.");
 
+    using Latin1Range = mozilla::Range<const Latin1Char>;
+
     for (uint32_t i = 0; i < UNIT_STATIC_LIMIT; i++) {
         Latin1Char buffer[] = { Latin1Char(i), '\0' };
-        JSFlatString* s = NewStringCopyN<NoGC>(cx, buffer, 1);
+        JSFlatString* s = NewInlineString<NoGC>(cx, Latin1Range(buffer, 1));
         if (!s)
             return false;
         HashNumber hash = mozilla::HashString(buffer, 1);
@@ -801,7 +803,7 @@ StaticStrings::init(JSContext* cx)
 
     for (uint32_t i = 0; i < NUM_SMALL_CHARS * NUM_SMALL_CHARS; i++) {
         Latin1Char buffer[] = { FROM_SMALL_CHAR(i >> 6), FROM_SMALL_CHAR(i & 0x3F), '\0' };
-        JSFlatString* s = NewStringCopyN<NoGC>(cx, buffer, 2);
+        JSFlatString* s = NewInlineString<NoGC>(cx, Latin1Range(buffer, 2));
         if (!s)
             return false;
         HashNumber hash = mozilla::HashString(buffer, 2);
@@ -820,7 +822,7 @@ StaticStrings::init(JSContext* cx)
                                     Latin1Char('0' + ((i / 10) % 10)),
                                     Latin1Char('0' + (i % 10)),
                                     '\0' };
-            JSFlatString* s = NewStringCopyN<NoGC>(cx, buffer, 3);
+            JSFlatString* s = NewInlineString<NoGC>(cx, Latin1Range(buffer, 3));
             if (!s)
                 return false;
             HashNumber hash = mozilla::HashString(buffer, 3);
@@ -1029,10 +1031,31 @@ NewInlineStringDeflated(ExclusiveContext* cx, mozilla::Range<const char16_t> cha
     return str;
 }
 
+template <typename CharT>
+static MOZ_ALWAYS_INLINE JSFlatString*
+TryEmptyOrStaticString(ExclusiveContext* cx, const CharT* chars, size_t n)
+{
+    // Measurements on popular websites indicate empty strings are pretty common
+    // and most strings with length 1 or 2 are in the StaticStrings table. For
+    // length 3 strings that's only about 1%, so we check n <= 2.
+    if (n <= 2) {
+        if (n == 0)
+            return cx->emptyString();
+
+        if (JSFlatString* str = cx->staticStrings().lookup(chars, n))
+            return str;
+    }
+
+    return nullptr;
+}
+
 template <AllowGC allowGC>
 static JSFlatString*
 NewStringDeflated(ExclusiveContext* cx, const char16_t* s, size_t n)
 {
+    if (JSFlatString* str = TryEmptyOrStaticString(cx, s, n))
+        return str;
+
     if (JSInlineString::lengthFits<Latin1Char>(n))
         return NewInlineStringDeflated<allowGC>(cx, mozilla::Range<const char16_t>(s, n));
 
@@ -1065,14 +1088,11 @@ template <AllowGC allowGC, typename CharT>
 JSFlatString*
 js::NewStringDontDeflate(ExclusiveContext* cx, CharT* chars, size_t length)
 {
-    if (length == 1) {
-        char16_t c = chars[0];
-        if (StaticStrings::hasUnit(c)) {
-            // Free |chars| because we're taking possession of it, but it's no
-            // longer needed because we use the static string instead.
-            js_free(chars);
-            return cx->staticStrings().getUnit(c);
-        }
+    if (JSFlatString* str = TryEmptyOrStaticString(cx, chars, length)) {
+        // Free |chars| because we're taking possession of it, but it's no
+        // longer needed because we use the static string instead.
+        js_free(chars);
+        return str;
     }
 
     if (JSInlineString::lengthFits<CharT>(length)) {
@@ -1105,14 +1125,6 @@ JSFlatString*
 js::NewString(ExclusiveContext* cx, CharT* chars, size_t length)
 {
     if (IsSame<CharT, char16_t>::value && CanStoreCharsAsLatin1(chars, length)) {
-        if (length == 1) {
-            char16_t c = chars[0];
-            if (StaticStrings::hasUnit(c)) {
-                js_free(chars);
-                return cx->staticStrings().getUnit(c);
-            }
-        }
-
         JSFlatString* s = NewStringDeflated<allowGC>(cx, chars, length);
         if (!s)
             return nullptr;
@@ -1143,6 +1155,9 @@ template <AllowGC allowGC, typename CharT>
 JSFlatString*
 NewStringCopyNDontDeflate(ExclusiveContext* cx, const CharT* s, size_t n)
 {
+    if (JSFlatString* str = TryEmptyOrStaticString(cx, s, n))
+        return str;
+
     if (JSInlineString::lengthFits<CharT>(n))
         return NewInlineString<allowGC>(cx, mozilla::Range<const CharT>(s, n));