diff --git a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
index 20b55ea150c3..1d9a70cb281a 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
@@ -37,6 +37,8 @@ nsUrlClassifierPrefixSet::Init(const nsACString& aName) {
 }
 
 nsUrlClassifierPrefixSet::~nsUrlClassifierPrefixSet() {
+  MutexAutoLock lock(mLock);
+
   for (uint32_t i = 0; i < mIndexDeltas.Length(); i++) {
     mIndexDeltas[i].Clear();
   }
diff --git a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
index dac631359e8f..3a9f22c587b2 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
@@ -51,7 +51,7 @@ class nsUrlClassifierPrefixSet final : public nsIUrlClassifierPrefixSet {
   friend class mozilla::safebrowsing::VariableLengthPrefixSet;
 
  private:
-  virtual ~nsUrlClassifierPrefixSet();
+  virtual ~nsUrlClassifierPrefixSet() MOZ_REQUIRES(mLock);
 
   static const uint32_t DELTAS_LIMIT = 120;
   static const uint32_t MAX_INDEX_DIFF = (1 << 16);