diff --git a/memory/build/PHC.cpp b/memory/build/PHC.cpp
index f6e02b2c22d2..94cb5596a501 100644
--- a/memory/build/PHC.cpp
+++ b/memory/build/PHC.cpp
@@ -343,7 +343,15 @@ static const Time kMaxTime = ~(Time(0));
 constexpr Delay Rnd64ToDelay(Delay aAvgDelay, uint64_t aRnd) {
   MOZ_ASSERT(IsPowerOfTwo(aAvgDelay), "must be a power of two");
 
-  return (aRnd & (aAvgDelay * 2 - 1)) + 1;
+  return (aRnd & (uint64_t(aAvgDelay) * 2 - 1)) + 1;
+}
+
+static Delay CheckProbability(int64_t aProb) {
+  // Limit delays calculated from prefs to 0x80000000, this is the largest
+  // power-of-two that fits in a Delay since it is a uint32_t.
+  // The minimum is 2 that way not every allocation goes straight to PHC.
+  return RoundUpPow2(
+      std::min(std::max(aProb, int64_t(2)), int64_t(0x80000000)));
 }
 
 // Maps a pointer to a PHC-specific structure:
@@ -863,6 +871,15 @@ class GMut {
     mPhcState = aState;
   }
 
+  void SetProbabilities(int64_t aAvgDelayFirst, int64_t aAvgDelayNormal,
+                        int64_t aAvgDelayPageReuse) {
+    MutexAutoLock lock(GMut::sMutex);
+
+    mAvgFirstAllocDelay = CheckProbability(aAvgDelayFirst);
+    mAvgAllocDelay = CheckProbability(aAvgDelayNormal);
+    mAvgPageReuseDelay = CheckProbability(aAvgDelayPageReuse);
+  }
+
  private:
   template <int N>
   uint64_t RandomSeed() {
@@ -1807,4 +1824,14 @@ void SetPHCState(PHCState aState) {
 
   gMut->SetState(aState);
 }
+
+void SetPHCProbabilities(int64_t aAvgDelayFirst, int64_t aAvgDelayNormal,
+                         int64_t aAvgDelayPageReuse) {
+  if (!maybe_init()) {
+    return;
+  }
+
+  gMut->SetProbabilities(aAvgDelayFirst, aAvgDelayNormal, aAvgDelayPageReuse);
+}
+
 }  // namespace mozilla::phc
diff --git a/memory/build/PHC.h b/memory/build/PHC.h
index ce5f7994e2d2..78820a5cf3f3 100644
--- a/memory/build/PHC.h
+++ b/memory/build/PHC.h
@@ -129,6 +129,10 @@ enum PHCState {
 
 MOZ_JEMALLOC_API void SetPHCState(PHCState aState);
 
+MOZ_JEMALLOC_API void SetPHCProbabilities(int64_t aAvgDelayFirst,
+                                          int64_t aAvgDelayNormal,
+                                          int64_t aAvgDelayPageReuse);
+
 struct MemoryUsage {
   // The amount of memory used for PHC metadata, eg information about each
   // allocation including stacks.
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index d4e9f1fbd0d7..c1e33bf3cb19 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -11051,6 +11051,21 @@
   value: 8000
   mirror: always
 
+- name: memory.phc.avg_delay.first
+  type: uint32_t
+  value: 65536
+  mirror: always
+
+- name: memory.phc.avg_delay.normal
+  type: uint32_t
+  value: 16384
+  mirror: always
+
+- name: memory.phc.avg_delay.page_reuse
+  type: uint32_t
+  value: 262144
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "midi."
 #---------------------------------------------------------------------------
diff --git a/toolkit/components/nimbus/FeatureManifest.yaml b/toolkit/components/nimbus/FeatureManifest.yaml
index 376c4f110015..9dd069455aa8 100644
--- a/toolkit/components/nimbus/FeatureManifest.yaml
+++ b/toolkit/components/nimbus/FeatureManifest.yaml
@@ -2288,6 +2288,24 @@ phc:
       setPref:
         branch: user
         pref: memory.phc.min_ram_mb
+    phcAvgDelayFirst:
+      description: The delay before the first PHC allocation
+      type: int
+      setPref:
+        branch: user
+        pref: memory.phc.avg_delay.first
+    phcAvgDelayNormal:
+      description: The delay between PHC allocations
+      type: int
+      setPref:
+        branch: user
+        pref: memory.phc.avg_delay.normal
+    phcAvgDelayPageReuse:
+      description: The delay before reusing a PHC page
+      type: int
+      setPref:
+        branch: user
+        pref: memory.phc.avg_delay.page_reuse
 
 mailto:
   description: Prefs to control aspects of the mailto handler
diff --git a/xpcom/base/PHCManager.cpp b/xpcom/base/PHCManager.cpp
index 7caccddf7d6f..f8124312f5f5 100644
--- a/xpcom/base/PHCManager.cpp
+++ b/xpcom/base/PHCManager.cpp
@@ -19,6 +19,9 @@ using namespace phc;
 
 static const char kPHCEnabledPref[] = "memory.phc.enabled";
 static const char kPHCMinRamMBPref[] = "memory.phc.min_ram_mb";
+static const char kPHCAvgDelayFirst[] = "memory.phc.avg_delay.first";
+static const char kPHCAvgDelayNormal[] = "memory.phc.avg_delay.normal";
+static const char kPHCAvgDelayPageRuse[] = "memory.phc.avg_delay.page_reuse";
 
 // True if PHC has ever been enabled for this process.
 static bool sWasPHCEnabled = false;
@@ -32,6 +35,12 @@ static void UpdatePHCState() {
   // slightly lower actual RAM available after some hardware devices
   // reserve some.
   if (StaticPrefs::memory_phc_enabled() && mem_size >= min_mem_size) {
+    // Set PHC probablities before enabling PHC so that the first allocation
+    // delay gets used.
+    SetPHCProbabilities(StaticPrefs::memory_phc_avg_delay_first(),
+                        StaticPrefs::memory_phc_avg_delay_normal(),
+                        StaticPrefs::memory_phc_avg_delay_page_reuse());
+
     SetPHCState(Enabled);
     sWasPHCEnabled = true;
   } else {
@@ -41,7 +50,10 @@ static void UpdatePHCState() {
 
 static void PrefChangeCallback(const char* aPrefName, void* aNull) {
   MOZ_ASSERT((0 == strcmp(aPrefName, kPHCEnabledPref)) ||
-             (0 == strcmp(aPrefName, kPHCMinRamMBPref)));
+             (0 == strcmp(aPrefName, kPHCMinRamMBPref)) ||
+             (0 == strcmp(aPrefName, kPHCAvgDelayFirst)) ||
+             (0 == strcmp(aPrefName, kPHCAvgDelayNormal)) ||
+             (0 == strcmp(aPrefName, kPHCAvgDelayPageRuse)));
 
   UpdatePHCState();
 }
@@ -49,6 +61,9 @@ static void PrefChangeCallback(const char* aPrefName, void* aNull) {
 void InitPHCState() {
   Preferences::RegisterCallback(PrefChangeCallback, kPHCEnabledPref);
   Preferences::RegisterCallback(PrefChangeCallback, kPHCMinRamMBPref);
+  Preferences::RegisterCallback(PrefChangeCallback, kPHCAvgDelayFirst);
+  Preferences::RegisterCallback(PrefChangeCallback, kPHCAvgDelayNormal);
+  Preferences::RegisterCallback(PrefChangeCallback, kPHCAvgDelayPageRuse);
   UpdatePHCState();
 }