diff --git a/parser/html/nsHtml5AtomTable.cpp b/parser/html/nsHtml5AtomTable.cpp
index 06bad88bf477..b6db18409850 100644
--- a/parser/html/nsHtml5AtomTable.cpp
+++ b/parser/html/nsHtml5AtomTable.cpp
@@ -11,21 +11,19 @@ nsHtml5AtomTable::nsHtml5AtomTable() : mRecentlyUsedParserAtoms{} {
 #endif
 }
 
-nsHtml5AtomTable::~nsHtml5AtomTable() {}
+nsHtml5AtomTable::~nsHtml5AtomTable() = default;
 
 nsAtom* nsHtml5AtomTable::GetAtom(const nsAString& aKey) {
-#ifdef DEBUG
   MOZ_ASSERT(mPermittedLookupEventTarget->IsOnCurrentThread());
-#endif
-
-  uint32_t index = mozilla::HashString(aKey) % RECENTLY_USED_PARSER_ATOMS_SIZE;
+  uint32_t hash = mozilla::HashString(aKey);
+  uint32_t index = hash % RECENTLY_USED_PARSER_ATOMS_SIZE;
   if (nsAtom* atom = mRecentlyUsedParserAtoms[index]) {
-    if (atom->Equals(aKey)) {
+    if (atom->hash() == hash && atom->Equals(aKey)) {
       return atom;
     }
   }
 
-  RefPtr<nsAtom> atom = NS_Atomize(aKey);
+  RefPtr<nsAtom> atom = NS_Atomize(aKey, hash);
   nsAtom* ret = atom.get();
   mRecentlyUsedParserAtoms[index] = std::move(atom);
   return ret;
diff --git a/xpcom/ds/nsAtom.h b/xpcom/ds/nsAtom.h
index 70cc0446fe0a..aa416e81ef49 100644
--- a/xpcom/ds/nsAtom.h
+++ b/xpcom/ds/nsAtom.h
@@ -247,6 +247,11 @@ already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String);
 // Find an atom that matches the given UTF-16 string. Never returns null.
 already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String);
 
+// Find an atom that matches the given UTF-16 string, with a known
+// already-computed hash via mozilla::HashString. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String,
+                                    uint32_t aKnownHash);
+
 // An optimized version of the method above for the main thread.
 already_AddRefed<nsAtom> NS_AtomizeMainThread(const nsAString& aUTF16String);
 
diff --git a/xpcom/ds/nsAtomTable.cpp b/xpcom/ds/nsAtomTable.cpp
index 8078e42076ea..c239ca0a53b9 100644
--- a/xpcom/ds/nsAtomTable.cpp
+++ b/xpcom/ds/nsAtomTable.cpp
@@ -11,6 +11,7 @@
 #include "mozilla/MruCache.h"
 #include "mozilla/RWLock.h"
 #include "mozilla/TextUtils.h"
+#include "nsHashKeys.h"
 #include "nsThreadUtils.h"
 
 #include "nsAtom.h"
@@ -141,9 +142,16 @@ struct AtomTableKey {
     MOZ_ASSERT(HashString(mUTF16String, mLength) == mHash);
   }
 
+  AtomTableKey(const char16_t* aUTF16String, uint32_t aLength, uint32_t aHash)
+      : mUTF16String(aUTF16String),
+        mUTF8String(nullptr),
+        mLength(aLength),
+        mHash(aHash) {
+    MOZ_ASSERT(HashString(mUTF16String, mLength) == mHash);
+  }
+
   AtomTableKey(const char16_t* aUTF16String, uint32_t aLength)
-      : mUTF16String(aUTF16String), mUTF8String(nullptr), mLength(aLength) {
-    mHash = HashString(mUTF16String, mLength);
+      : AtomTableKey(aUTF16String, aLength, HashString(aUTF16String, aLength)) {
   }
 
   AtomTableKey(const char* aUTF8String, uint32_t aLength, bool* aErr)
@@ -209,7 +217,8 @@ class nsAtomTable {
   nsAtomSubTable& SelectSubTable(AtomTableKey& aKey);
   void AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf, AtomsSizes& aSizes);
   void GC(GCKind aKind);
-  already_AddRefed<nsAtom> Atomize(const nsAString& aUTF16String);
+  already_AddRefed<nsAtom> Atomize(const nsAString& aUTF16String,
+                                   uint32_t aHash);
   already_AddRefed<nsAtom> Atomize(const nsACString& aUTF8String);
   already_AddRefed<nsAtom> AtomizeMainThread(const nsAString& aUTF16String);
   nsStaticAtom* GetStaticAtom(const nsAString& aUTF16String);
@@ -547,7 +556,7 @@ already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsACString& aUTF8String) {
     // and atomize the result.
     nsString str;
     CopyUTF8toUTF16(aUTF8String, str);
-    return Atomize(str);
+    return Atomize(str, HashString(str));
   }
   nsAtomSubTable& table = SelectSubTable(key);
   {
@@ -580,12 +589,12 @@ already_AddRefed<nsAtom> NS_Atomize(const nsACString& aUTF8String) {
 }
 
 already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String) {
-  MOZ_ASSERT(gAtomTable);
-  return gAtomTable->Atomize(nsDependentString(aUTF16String));
+  return NS_Atomize(nsDependentString(aUTF16String));
 }
 
-already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsAString& aUTF16String) {
-  AtomTableKey key(aUTF16String.Data(), aUTF16String.Length());
+already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsAString& aUTF16String,
+                                              uint32_t aHash) {
+  AtomTableKey key(aUTF16String.Data(), aUTF16String.Length(), aHash);
   nsAtomSubTable& table = SelectSubTable(key);
   {
     AutoReadLock lock(table.mLock);
@@ -608,9 +617,14 @@ already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsAString& aUTF16String) {
   return atom.forget();
 }
 
-already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String) {
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String,
+                                    uint32_t aKnownHash) {
   MOZ_ASSERT(gAtomTable);
-  return gAtomTable->Atomize(aUTF16String);
+  return gAtomTable->Atomize(aUTF16String, aKnownHash);
+}
+
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String) {
+  return NS_Atomize(aUTF16String, HashString(aUTF16String));
 }
 
 already_AddRefed<nsAtom> nsAtomTable::AtomizeMainThread(