--- a/toolkit/components/url-classifier/Classifier.cpp
+++ b/toolkit/components/url-classifier/Classifier.cpp
@@ -871,16 +871,21 @@ Classifier::ApplyUpdatesForeground(nsres
     ResetTables(Clear_All, nsTArray<nsCString> { nsCString(aFailedTableName) });
   }
   return aBackgroundRv;
 }
 
 nsresult
 Classifier::ApplyFullHashes(nsTArray<TableUpdate*>* aUpdates)
 {
+  MOZ_ASSERT(NS_GetCurrentThread() != mUpdateThread,
+             "ApplyFullHashes() MUST NOT be called on update thread");
+  MOZ_ASSERT(!NS_IsMainThread(),
+             "ApplyFullHashes() must be called on the classifier worker thread.");
+
   LOG(("Applying %zu table gethashes.", aUpdates->Length()));
 
   ScopedUpdatesClearer scopedUpdatesClearer(aUpdates);
   for (uint32_t i = 0; i < aUpdates->Length(); i++) {
     TableUpdate *update = aUpdates->ElementAt(i);
 
     nsresult rv = UpdateCache(update);
     NS_ENSURE_SUCCESS(rv, rv);