diff --git a/toolkit/components/extensions/MatchPattern.cpp b/toolkit/components/extensions/MatchPattern.cpp
index 13d117e9171f..6a627650d675 100644
--- a/toolkit/components/extensions/MatchPattern.cpp
+++ b/toolkit/components/extensions/MatchPattern.cpp
@@ -119,7 +119,7 @@ nsAtom* URLInfo::Scheme() const {
   if (!mScheme) {
     nsCString scheme;
     if (NS_SUCCEEDED(mURI->GetScheme(scheme))) {
-      mScheme = NS_AtomizeMainThread(NS_ConvertASCIItoUTF16(scheme));
+      mScheme = NS_Atomize(scheme);
     }
   }
   return mScheme;
@@ -196,6 +196,7 @@ bool URLInfo::InheritsPrincipal() const {
 }
 
 bool URLInfo::IsNonOpaqueURL() const {
+  MOZ_ASSERT(NS_IsMainThread());
   if (!mIsNonOpaqueURL.isSome()) {
     RefPtr<AtomSet> nonOpaqueSchemes = NonOpaqueSchemes();
     mIsNonOpaqueURL.emplace(nonOpaqueSchemes->Contains(Scheme()));
diff --git a/toolkit/components/extensions/MatchPattern.h b/toolkit/components/extensions/MatchPattern.h
index 67a051648dc3..17f7b717e3dd 100644
--- a/toolkit/components/extensions/MatchPattern.h
+++ b/toolkit/components/extensions/MatchPattern.h
@@ -112,6 +112,8 @@ class URLInfo final {
   // that document were to have an opaque origin (null principal).
   // These URLs are more verbose than the precursor (origin) of a null
   // principal, and therefore preferred for matching purposes.
+  //
+  // NOTE: This method is main-thread only
   bool IsNonOpaqueURL() const;
 
  private: