diff --git a/xpcom/glue/moz.build b/xpcom/glue/moz.build
index b07fea4a8932..0b296745023c 100644
--- a/xpcom/glue/moz.build
+++ b/xpcom/glue/moz.build
@@ -112,3 +112,5 @@ LOCAL_INCLUDES += [
 
 if CONFIG['ENABLE_TESTS']:
     DIRS += ['tests/gtest']
+
+FAIL_ON_WARNINGS = True
diff --git a/xpcom/glue/nomozalloc/moz.build b/xpcom/glue/nomozalloc/moz.build
index 179ccfa0ed98..d74185497368 100644
--- a/xpcom/glue/nomozalloc/moz.build
+++ b/xpcom/glue/nomozalloc/moz.build
@@ -40,3 +40,5 @@ USE_STATIC_LIBS = True
 
 # Don't use STL wrappers here (i.e. wrapped <new>); they require mozalloc
 DISABLE_STL_WRAPPING = True
+
+FAIL_ON_WARNINGS = True
diff --git a/xpcom/glue/pldhash.h b/xpcom/glue/pldhash.h
index f5dad9cbdc82..2086ef27e44f 100644
--- a/xpcom/glue/pldhash.h
+++ b/xpcom/glue/pldhash.h
@@ -184,9 +184,12 @@ private:
    * |mRecursionLevel| is only used in debug builds, but is present in opt
    * builds to avoid binary compatibility problems when mixing DEBUG and
    * non-DEBUG components.  (Actually, even if it were removed,
-   * sizeof(PLDHashTable) wouldn't change, due to struct padding.)
+   * sizeof(PLDHashTable) wouldn't change, due to struct padding.) Make it
+   * protected to suppress -Wunused-private-field warnings in opt builds.
    */
+protected:
   mutable uint16_t    mRecursionLevel;/* used to detect unsafe re-entry */
+private:
   uint32_t            mEntrySize;     /* number of bytes in an entry */
   uint32_t            mEntryCount;    /* number of entries in table */
   uint32_t            mRemovedCount;  /* removed entry sentinels in table */
diff --git a/xpcom/glue/standalone/moz.build b/xpcom/glue/standalone/moz.build
index 84c9da2fa8d6..9942a3be49f7 100644
--- a/xpcom/glue/standalone/moz.build
+++ b/xpcom/glue/standalone/moz.build
@@ -42,3 +42,5 @@ LOCAL_INCLUDES += [
 
 # Don't use STL wrappers here (i.e. wrapped <new>); they require mozalloc
 DISABLE_STL_WRAPPING = True
+
+FAIL_ON_WARNINGS = True
diff --git a/xpcom/glue/standalone/staticruntime/moz.build b/xpcom/glue/standalone/staticruntime/moz.build
index caf310015533..a4c84e56787c 100644
--- a/xpcom/glue/standalone/staticruntime/moz.build
+++ b/xpcom/glue/standalone/staticruntime/moz.build
@@ -37,3 +37,5 @@ USE_STATIC_LIBS = True
 
 # Don't use STL wrappers here (i.e. wrapped <new>); they require mozalloc
 DISABLE_STL_WRAPPING = True
+
+FAIL_ON_WARNINGS = True
diff --git a/xpcom/glue/staticruntime/moz.build b/xpcom/glue/staticruntime/moz.build
index 33cba07ec67d..86b17efd1b35 100644
--- a/xpcom/glue/staticruntime/moz.build
+++ b/xpcom/glue/staticruntime/moz.build
@@ -35,3 +35,5 @@ USE_STATIC_LIBS = True
 
 # Don't use STL wrappers here (i.e. wrapped <new>); they require mozalloc
 DISABLE_STL_WRAPPING = True
+
+FAIL_ON_WARNINGS = True