diff --git a/dom/base/nsDOMTokenList.cpp b/dom/base/nsDOMTokenList.cpp
index 87fb11e8c7..02acabacac 100644
--- a/dom/base/nsDOMTokenList.cpp
+++ b/dom/base/nsDOMTokenList.cpp
@@ -127,7 +127,29 @@ void
 nsDOMTokenList::AddInternal(const nsAttrValue* aAttr,
                             const nsTArray<nsString>& aTokens)
 {
-  if (!mElement) {
+  if (!mElement || aTokens.IsEmpty()) {
+    return;
+  }
+
+  // Hot path: single-token classList.add() when the class is already present.
+  // Skip attribute rebuild and SetAttr entirely (avoids mutation observers /
+  // style invalidation). Frameworks hit this constantly.
+  if (aTokens.Length() == 1) {
+    const nsString& token = aTokens[0];
+    if (aAttr && aAttr->Contains(token)) {
+      return;
+    }
+
+    nsAutoString resultStr;
+    if (aAttr) {
+      aAttr->ToString(resultStr);
+      if (!resultStr.IsEmpty() &&
+          !nsContentUtils::IsHTMLWhitespace(resultStr.Last())) {
+        resultStr.Append(' ');
+      }
+    }
+    resultStr.Append(token);
+    mElement->SetAttr(kNameSpaceID_None, mAttrAtom, resultStr, true);
     return;
   }
 
@@ -161,6 +183,11 @@ nsDOMTokenList::AddInternal(const nsAttrValue* aAttr,
     addedClasses.AppendElement(aToken);
   }
 
+  // No new tokens: leave the attribute untouched (do not create class="").
+  if (!oneWasAdded) {
+    return;
+  }
+
   mElement->SetAttr(kNameSpaceID_None, mAttrAtom, resultStr, true);
 }
 
@@ -190,20 +217,37 @@ nsDOMTokenList::RemoveInternal(const nsAttrValue* aAttr,
 {
   MOZ_ASSERT(aAttr, "Need an attribute");
 
+  if (aTokens.IsEmpty()) {
+    return;
+  }
+
+  // Hot path: single-token classList.remove() when the class is absent.
+  if (aTokens.Length() == 1 && !aAttr->Contains(aTokens[0])) {
+    return;
+  }
+
   nsAutoString input;
   aAttr->ToString(input);
 
   WhitespaceTokenizer tokenizer(input);
   nsAutoString output;
+  bool removed = false;
 
   while (tokenizer.hasMoreTokens()) {
     auto& currentToken = tokenizer.nextToken();
-    if (!aTokens.Contains(currentToken)) {
-      if (!output.IsEmpty()) {
-        output.Append(char16_t(' '));
-      }
-      output.Append(currentToken);
+    if (aTokens.Contains(currentToken)) {
+      removed = true;
+      continue;
     }
+    if (!output.IsEmpty()) {
+      output.Append(char16_t(' '));
+    }
+    output.Append(currentToken);
+  }
+
+  // Token set unchanged: skip SetAttr to avoid style/mutation work.
+  if (!removed) {
+    return;
   }
 
   mElement->SetAttr(kNameSpaceID_None, mAttrAtom, output, true);
@@ -302,6 +346,16 @@ nsDOMTokenList::ReplaceInternal(const nsAttrValue* aAttr,
                                 const nsAString& aToken,
                                 const nsAString& aNewToken)
 {
+  // Replacing a token with itself cannot change the token set.
+  if (aToken.Equals(aNewToken)) {
+    return;
+  }
+
+  // Old token absent: no update (and avoid rewriting whitespace).
+  if (!aAttr->Contains(aToken)) {
+    return;
+  }
+
   nsAutoString attribute;
   aAttr->ToString(attribute);
 
diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 7fa01b93a4..6f646b3b2d 100644
--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@@ -1221,7 +1221,12 @@ js::array_join(JSContext* cx, unsigned argc, Value* vp)
         sepstr = cx->names().comma;
     }
 
-    // Step 6 is implicit in the loops below.
+    // Step 6: empty arrays always join to the empty string.
+    // (Separator ToString above still runs for side effects / OOM.)
+    if (length == 0) {
+        args.rval().setString(cx->names().empty);
+        return true;
+    }
 
     // An optimized version of a special case of steps 7-11: when length==1 and
     // the 0th element is a string, ToString() of that element is a no-op and
@@ -1381,8 +1386,8 @@ template <JSValueType Type>
 DenseElementResult
 ArrayReverseDenseKernel(JSContext* cx, HandleObject obj, uint32_t length)
 {
-    /* An empty array or an array with no elements is already reversed. */
-    if (length == 0 || GetBoxedOrUnboxedInitializedLength<Type>(obj) == 0)
+    /* Empty, singleton, or uninitialized arrays are already reversed. */
+    if (length <= 1 || GetBoxedOrUnboxedInitializedLength<Type>(obj) == 0)
         return DenseElementResult::Success;
 
     if (Type == JSVAL_TYPE_MAGIC) {
@@ -1451,6 +1456,12 @@ js::array_reverse(JSContext* cx, unsigned argc, Value* vp)
     if (!GetLengthProperty(cx, obj, &len))
         return false;
 
+    // length 0/1: reverse is a no-op; return this immediately.
+    if (len <= 1) {
+        args.rval().setObject(*obj);
+        return true;
+    }
+
     if (!ObjectMayHaveExtraIndexedProperties(obj)) {
         ArrayReverseDenseKernelFunctor functor(cx, obj, len);
         DenseElementResult result = CallBoxedOrUnboxedSpecialization(functor, obj);
diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index aa0a503f72..5209e979de 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -2329,6 +2329,12 @@ js::str_startsWith(JSContext* cx, unsigned argc, Value* vp)
     // Step 13
     uint32_t searchLen = searchStr->length();
 
+    // Empty search string always matches (and is extremely common).
+    if (searchLen == 0) {
+        args.rval().setBoolean(true);
+        return true;
+    }
+
     // Step 14
     if (searchLen + start < searchLen || searchLen + start > textLen) {
         args.rval().setBoolean(false);
@@ -2395,6 +2401,12 @@ js::str_endsWith(JSContext* cx, unsigned argc, Value* vp)
     // Step 13
     uint32_t searchLen = searchStr->length();
 
+    // Empty search string always matches.
+    if (searchLen == 0) {
+        args.rval().setBoolean(true);
+        return true;
+    }
+
     // Step 15 (reordered)
     if (searchLen > end) {
         args.rval().setBoolean(false);