--- a/memory/replace/dmd/DMD.cpp
+++ b/memory/replace/dmd/DMD.cpp
@@ -1251,17 +1251,17 @@ FreeCallback(void* aPtr, Thread* aT, Dea
     GCStackTraces();
   }
 }
 
 //---------------------------------------------------------------------------
 // malloc/free interception
 //---------------------------------------------------------------------------
 
-static void Init(malloc_table_t* aMallocTable);
+static bool Init(malloc_table_t* aMallocTable);
 
 } // namespace dmd
 } // namespace mozilla
 
 static void*
 replace_malloc(size_t aSize)
 {
   using namespace mozilla::dmd;
@@ -1363,21 +1363,22 @@ replace_free(void* aPtr)
   FreeCallback(aPtr, t, &db);
   MaybeAddToDeadBlockTable(db);
   gMallocTable.free(aPtr);
 }
 
 void
 replace_init(malloc_table_t* aMallocTable, ReplaceMallocBridge** aBridge)
 {
-  mozilla::dmd::Init(aMallocTable);
+  if (mozilla::dmd::Init(aMallocTable)) {
 #define MALLOC_FUNCS MALLOC_FUNCS_MALLOC_BASE
 #define MALLOC_DECL(name, ...) aMallocTable->name = replace_ ## name;
 #include "malloc_decls.h"
-  *aBridge = mozilla::dmd::gDMDBridge;
+    *aBridge = mozilla::dmd::gDMDBridge;
+  }
 }
 
 namespace mozilla {
 namespace dmd {
 
 //---------------------------------------------------------------------------
 // Options (Part 2)
 //---------------------------------------------------------------------------
@@ -1434,19 +1435,16 @@ Options::GetBool(const char* aArg, const
 
 Options::Options(const char* aDMDEnvVar)
   : mDMDEnvVar(aDMDEnvVar ? InfallibleAllocPolicy::strdup_(aDMDEnvVar)
                           : nullptr)
   , mMode(Mode::DarkMatter)
   , mStacks(Stacks::Partial)
   , mShowDumpStats(false)
 {
-  // It's no longer necessary to set the DMD env var to "1" if you want default
-  // options (you can leave it undefined) but we still accept "1" for
-  // backwards compatibility.
   char* e = mDMDEnvVar;
   if (e && strcmp(e, "1") != 0) {
     bool isEnd = false;
     while (!isEnd) {
       // Consume leading whitespace.
       while (isspace(*e)) {
         e++;
       }
@@ -1546,41 +1544,42 @@ postfork()
     gStateLock->Unlock();
   }
 }
 
 // WARNING: this function runs *very* early -- before all static initializers
 // have run.  For this reason, non-scalar globals such as gStateLock and
 // gStackTraceTable are allocated dynamically (so we can guarantee their
 // construction in this function) rather than statically.
-static void
+static bool
 Init(malloc_table_t* aMallocTable)
 {
+  // DMD is controlled by the |DMD| environment variable.
+  const char* e = getenv("DMD");
+
+  if (!e) {
+    return false;
+  }
+  // Initialize the function table first, because StatusMsg uses
+  // InfallibleAllocPolicy::malloc_, which uses it.
   gMallocTable = *aMallocTable;
+
+  StatusMsg("$DMD = '%s'\n", e);
+
   gDMDBridge = InfallibleAllocPolicy::new_<DMDBridge>();
 
 #ifndef XP_WIN
   // Avoid deadlocks when forking by acquiring our state lock prior to forking
   // and releasing it after forking. See |LogAlloc|'s |replace_init| for
   // in-depth details.
   //
   // Note: This must run after attempting an allocation so as to give the
   // system malloc a chance to insert its own atfork handler.
   pthread_atfork(prefork, postfork, postfork);
 #endif
-
-  // DMD is controlled by the |DMD| environment variable.
-  const char* e = getenv("DMD");
-
-  if (e) {
-    StatusMsg("$DMD = '%s'\n", e);
-  } else {
-    StatusMsg("$DMD is undefined\n");
-  }
-
   // Parse $DMD env var.
   gOptions = InfallibleAllocPolicy::new_<Options>(e);
 
   gStateLock = InfallibleAllocPolicy::new_<Mutex>();
 
   gBernoulli = (FastBernoulliTrial*)
     InfallibleAllocPolicy::malloc_(sizeof(FastBernoulliTrial));
   ResetBernoulli();
@@ -1599,16 +1598,17 @@ Init(malloc_table_t* aMallocTable)
     // Create this even if the mode isn't Cumulative (albeit with a small
     // size), in case the mode is changed later on (as is done by SmokeDMD.cpp,
     // for example).
     gDeadBlockTable = InfallibleAllocPolicy::new_<DeadBlockTable>();
     size_t tableSize = gOptions->IsCumulativeMode() ? 8192 : 4;
     MOZ_ALWAYS_TRUE(gDeadBlockTable->init(tableSize));
   }
 
+  return true;
 }
 
 //---------------------------------------------------------------------------
 // Block reporting and unreporting
 //---------------------------------------------------------------------------
 
 static void
 ReportHelper(const void* aPtr, bool aReportedOnAlloc)

--- a/memory/replace/dmd/test/script-diff-dark-matter-expected.txt
+++ b/memory/replace/dmd/test/script-diff-dark-matter-expected.txt
@@ -2,17 +2,17 @@
 # dmd.py --filter-stacks-for-testing -o script-diff-dark-matter-actual.txt script-diff-dark-matter1.json script-diff-dark-matter2.json
 
 Invocation 1 {
   $DMD = '--mode=dark-matter'
   Mode = 'dark-matter'
 }
 
 Invocation 2 {
-  $DMD is undefined
+  $DMD = '1'
   Mode = 'dark-matter'
 }
 
 #-----------------------------------------------------------------
 
 Twice-reported {
   -1 blocks in heap block record 1 of 1
   -1,088 bytes (-1,064 requested / -24 slop)

--- a/memory/replace/dmd/test/script-diff-dark-matter2.json
+++ b/memory/replace/dmd/test/script-diff-dark-matter2.json
@@ -1,12 +1,12 @@
 {
  "version": 5,
  "invocation": {
-  "dmdEnvVar": null,
+  "dmdEnvVar": "1",
   "mode": "dark-matter"
  },
  "blockList": [
   {"req": 4096, "alloc": "A", "num": 4},
 
   {"req": 8192, "alloc": "B"},
   {"req": 8192, "alloc": "B"},
 

--- a/memory/replace/dmd/test/script-ignore-alloc-fns-expected.txt
+++ b/memory/replace/dmd/test/script-ignore-alloc-fns-expected.txt
@@ -1,13 +1,13 @@
 #-----------------------------------------------------------------
 # dmd.py --filter-stacks-for-testing -o script-ignore-alloc-fns-actual.txt --ignore-alloc-fns script-ignore-alloc-fns.json
 
 Invocation {
-  $DMD is undefined
+  $DMD = '1'
   Mode = 'dark-matter'
 }
 
 #-----------------------------------------------------------------
 
 # no twice-reported heap blocks
 
 #-----------------------------------------------------------------

--- a/memory/replace/dmd/test/script-ignore-alloc-fns.json
+++ b/memory/replace/dmd/test/script-ignore-alloc-fns.json
@@ -1,12 +1,12 @@
 {
  "version": 5,
  "invocation": {
-  "dmdEnvVar": null,
+  "dmdEnvVar": "1",
   "mode": "dark-matter"
  },
  "blockList": [
   {"req": 1048576,           "alloc": "A"},
   {"req": 65536,             "alloc": "B"},
   {"req": 8000, "slop": 192, "alloc": "C"},
   {"req": 2500,              "alloc": "D"}
  ],

--- a/memory/replace/dmd/test/test_dmd.js
+++ b/memory/replace/dmd/test/test_dmd.js
@@ -131,16 +131,17 @@ function run_test() {
   // These tests do complete end-to-end testing of DMD, i.e. both the C++ code
   // that generates the JSON output, and the script that post-processes that
   // output.
   //
   // Run these synchronously, because test() updates the complete*.json files
   // in-place (to fix stacks) when it runs dmd.py, and that's not safe to do
   // asynchronously.
 
+  gEnv.set('DMD', '1');
   gEnv.set(gEnv.get("DMD_PRELOAD_VAR"), gEnv.get("DMD_PRELOAD_VALUE"));
 
   runProcess(gDmdTestFile, []);
 
   function test2(aTestName, aMode) {
     let name = "complete-" + aTestName + "-" + aMode;
     jsonFile = FileUtils.getFile("CurWorkD", [name + ".json"]);
     test(name, [jsonFile.path]);

--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -940,16 +940,18 @@ class RunProgram(MachCommandBase):
                     "MOZ_REPLACE_MALLOC_LIB": dmd_lib,
                 },
             }
 
             arch = self.substs['OS_ARCH']
 
             if dmd_params:
                 env_vars[arch]["DMD"] = " ".join(dmd_params)
+            else:
+                env_vars[arch]["DMD"] = "1"
 
             extra_env.update(env_vars.get(arch, {}))
 
         return self.run_process(args=args, ensure_exit_code=False,
             pass_thru=True, append_env=extra_env)
 
 @CommandProvider
 class Buildsymbols(MachCommandBase):

--- a/testing/awsy/mach_commands.py
+++ b/testing/awsy/mach_commands.py
@@ -165,16 +165,18 @@ class MachCommands(MachCommandBase):
                 "WINNT": {
                     "MOZ_REPLACE_MALLOC_LIB": dmd_lib,
                 },
             }
 
             arch = self.substs['OS_ARCH']
             for k, v in env_vars[arch].iteritems():
                 os.environ[k] = v
+            if 'DMD' not in os.environ:
+                os.environ['DMD'] = '1'
 
             # Also add the bin dir to the python path so we can use dmd.py
             if bin_dir not in sys.path:
                 sys.path.append(bin_dir)
 
         for k, v in kwargs.iteritems():
             setattr(args, k, v)
 

--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1629,16 +1629,19 @@ toolbar#nav-bar {
         if hasattr(options, "topsrcdir"):
             browserEnv["MOZ_DEVELOPER_REPO_DIR"] = options.topsrcdir
         if hasattr(options, "topobjdir"):
             browserEnv["MOZ_DEVELOPER_OBJ_DIR"] = options.topobjdir
 
         if options.headless:
             browserEnv["MOZ_HEADLESS"] = '1'
 
+        if options.dmd:
+            browserEnv["DMD"] = os.environ.get('DMD', '1')
+
         # These variables are necessary for correct application startup; change
         # via the commandline at your own risk.
         browserEnv["XPCOM_DEBUG_BREAK"] = "stack"
 
         # interpolate environment passed with options
         try:
             browserEnv.update(
                 dict(

--- a/testing/mochitest/runtestsremote.py
+++ b/testing/mochitest/runtestsremote.py
@@ -275,16 +275,18 @@ class MochiRemote(MochitestDesktop):
         # remove desktop environment not used on device
         if "XPCOM_MEM_BLOAT_LOG" in browserEnv:
             del browserEnv["XPCOM_MEM_BLOAT_LOG"]
         # override mozLogs to avoid processing in MochitestDesktop base class
         self.mozLogs = None
         browserEnv["MOZ_LOG_FILE"] = os.path.join(
             self.remoteMozLog,
             self.mozLogName)
+        if options.dmd:
+            browserEnv['DMD'] = '1'
         return browserEnv
 
     def runApp(self, *args, **kwargs):
         """front-end automation.py's `runApp` functionality until FennecRunner is written"""
 
         # automation.py/remoteautomation `runApp` takes the profile path,
         # whereas runtest.py's `runApp` takes a mozprofile object.
         if 'profileDir' not in kwargs and 'profile' in kwargs: