--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -268,19 +268,16 @@ private:
 struct PrefHashEntry : PLDHashEntryHdr
 {
   PrefTypeFlags mPrefFlags; // this field first to minimize 64-bit struct size
   const char* mKey;
   PrefValue mDefaultPref;
   PrefValue mUserPref;
 };
 
-static nsresult
-PREF_ClearUserPref(const char* aPrefName);
-
 static void
 ClearPrefEntry(PLDHashTable* aTable, PLDHashEntryHdr* aEntry)
 {
   auto pref = static_cast<PrefHashEntry*>(aEntry);
   if (pref->mPrefFlags.IsTypeString()) {
     free(const_cast<char*>(pref->mDefaultPref.mStringVal));
     free(const_cast<char*>(pref->mUserPref.mStringVal));
   }