diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 98f1041d9580..91cd2d77b4ba 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -613,7 +613,9 @@ public:
 class NodePool
 {
 private:
-  enum { BlockSize = 8 * 1024 }; // could be int template parameter
+  // The -2 allows us to use |BlockSize + 1| for |mEntries|, and fit |mNext|,
+  // all without causing slop.
+  enum { BlockSize = 8 * 1024 - 2 };
 
   struct Block
   {
@@ -623,6 +625,13 @@ private:
     Block()
     {
       NS_NOTREACHED("should never be called");
+
+      // Ensure Block is the right size (see the comment on BlockSize above).
+      static_assert(
+        sizeof(Block) == 163824 ||      // 32-bit; equals 39.997 pages
+        sizeof(Block) == 262120,        // 64-bit; equals 63.994 pages
+        "ill-sized NodePool::Block"
+      );
     }
     ~Block()
     {