--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -1073,18 +1073,16 @@ struct ArenaTreeTrait
 // - "private" arenas, used through the moz_arena_* API
 // - all the other arenas: the default arena, and thread-local arenas,
 //   used by the standard API.
 class ArenaCollection
 {
 public:
   bool Init()
   {
-    mArenas.Init();
-    mPrivateArenas.Init();
     mDefaultArena =
       mLock.Init() ? CreateArena(/* IsPrivate = */ false) : nullptr;
     if (mDefaultArena) {
       // arena_t constructor sets this to a lower value for thread local
       // arenas; Reset to the default value for the main arena.
       mDefaultArena->mMaxDirty = opt_dirty_max;
     }
     return bool(mDefaultArena);
@@ -2896,17 +2894,17 @@ arena_bin_t::Init(SizeClass aSizeClass)
   // Size of the run header, excluding mRegionsMask.
   static const size_t kRawHeaderSize = offsetof(arena_run_t, mRegionsMask);
 
   MOZ_ASSERT(aSizeClass.Size() <= gMaxBinClass);
 
   try_run_size = gPageSize;
 
   mCurrentRun = nullptr;
-  mNonFullRuns.Init();
+  new (&mNonFullRuns) RedBlackTree<arena_chunk_map_t, ArenaRunTreeTrait>();
   mSizeClass = aSizeClass.Size();
   mNumRuns = 0;
 
   // mRunSize expansion loop.
   while (true) {
     try_nregs = ((try_run_size - kRawHeaderSize) / mSizeClass) +
                 1; // Counter-act try_nregs-- in loop.
 
@@ -3742,29 +3740,26 @@ arena_t::arena_t()
   unsigned i;
 
   MOZ_RELEASE_ASSERT(mLock.Init());
 
   memset(&mLink, 0, sizeof(mLink));
   memset(&mStats, 0, sizeof(arena_stats_t));
 
   // Initialize chunks.
-  mChunksDirty.Init();
 #ifdef MALLOC_DOUBLE_PURGE
   new (&mChunksMAdvised) DoublyLinkedList<arena_chunk_t>();
 #endif
   mSpare = nullptr;
 
   mNumDirty = 0;
   // Reduce the maximum amount of dirty pages we allow to be kept on
   // thread local arenas. TODO: make this more flexible.
   mMaxDirty = opt_dirty_max >> 3;
 
-  mRunsAvail.Init();
-
   // Initialize bins.
   SizeClass sizeClass(1);
 
   for (i = 0;; i++) {
     arena_bin_t& bin = mBins[i];
     bin.Init(sizeClass);
 
     // SizeClass doesn't want sizes larger than gMaxSubPageClass for now.
@@ -4142,22 +4137,19 @@ static
       }
     }
   }
 
   gRecycledSize = 0;
 
   // Initialize chunks data.
   chunks_mtx.Init();
-  gChunksBySize.Init();
-  gChunksByAddress.Init();
 
   // Initialize huge allocation data.
   huge_mtx.Init();
-  huge.Init();
   huge_allocated = 0;
   huge_mapped = 0;
 
   // Initialize base allocation data structures.
   base_mapped = 0;
   base_committed = 0;
   base_nodes = nullptr;
   base_mtx.Init();

--- a/memory/build/rb.h
+++ b/memory/build/rb.h
@@ -117,17 +117,20 @@ public:
   }
 };
 
 // Tree structure.
 template<typename T, typename Trait>
 class RedBlackTree
 {
 public:
-  void Init() { mRoot = nullptr; }
+  constexpr RedBlackTree()
+    : mRoot(nullptr)
+  {
+  }
 
   T* First(T* aStart = nullptr)
   {
     return First(reinterpret_cast<TreeNode*>(aStart));
   }
 
   T* Last(T* aStart = nullptr)
   {