--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -747,17 +747,16 @@ void PresShell::RemoveWeakFrame(WeakFram
 
 already_AddRefed<nsFrameSelection> PresShell::FrameSelection() {
   RefPtr<nsFrameSelection> ret = mSelection;
   return ret.forget();
 }
 
 //----------------------------------------------------------------------
 
-static bool sSynthMouseMove = true;
 static uint32_t sNextPresShellId;
 
 /* static */
 bool PresShell::AccessibleCaretEnabled(nsIDocShell* aDocShell) {
   // If the pref forces it on, then enable it.
   if (StaticPrefs::layout_accessiblecaret_enabled()) {
     return true;
   }
@@ -844,22 +843,16 @@ PresShell::PresShell()
 
 #ifdef MOZ_REFLOW_PERF
   mReflowCountMgr = MakeUnique<ReflowCountMgr>();
   mReflowCountMgr->SetPresContext(mPresContext);
   mReflowCountMgr->SetPresShell(this);
 #endif
   mLastOSWake = mLoadBegin = TimeStamp::Now();
 
-  static bool addedSynthMouseMove = false;
-  if (!addedSynthMouseMove) {
-    Preferences::AddBoolVarCache(&sSynthMouseMove,
-                                 "layout.reflow.synthMouseMove", true);
-    addedSynthMouseMove = true;
-  }
   PointerEventHandler::Initialize();
 }
 
 NS_INTERFACE_TABLE_HEAD(PresShell)
   NS_INTERFACE_TABLE_BEGIN
     // In most cases, PresShell should be treated as concrete class, but need to
     // QI for weak reference.  Therefore, the case needed by do_QueryReferent()
     // should be tested first.
@@ -5331,17 +5324,17 @@ void PresShell::SetRenderingState(const 
     }
   }
 
   mRenderingStateFlags = aState.mRenderingStateFlags;
   mResolution = aState.mResolution;
 }
 
 void PresShell::SynthesizeMouseMove(bool aFromScroll) {
-  if (!sSynthMouseMove) return;
+  if (!StaticPrefs::layout_reflow_synthMouseMove()) return;
 
   if (mPaintingSuppressed || !mIsActive || !mPresContext) {
     return;
   }
 
   if (!mPresContext->IsRoot()) {
     if (PresShell* rootPresShell = GetRootPresShell()) {
       rootPresShell->SynthesizeMouseMove(aFromScroll);
@@ -9116,17 +9109,17 @@ void PresShell::DidDoReflow(bool aInterr
     DOMHighResTimeStamp now = GetPerformanceNowUnclamped();
     docShell->NotifyReflowObservers(aInterruptible, mLastReflowStart, now);
   }
 
   if (!mPresContext->HasPendingInterrupt()) {
     mDocument->ScheduleResizeObserversNotification();
   }
 
-  if (sSynthMouseMove) {
+  if (StaticPrefs::layout_reflow_synthMouseMove()) {
     SynthesizeMouseMove(false);
   }
 
   mPresContext->NotifyMissingFonts();
 }
 
 DOMHighResTimeStamp PresShell::GetPerformanceNowUnclamped() {
   DOMHighResTimeStamp now = 0;

--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -4427,16 +4427,21 @@
   value: 64
   mirror: always
 
 - name: layout.paint_rects_separately
   type: bool
   value: true
   mirror: once
 
+- name: layout.reflow.synthMouseMove
+  type: bool
+  value: true
+  mirror: always
+
 # This and code dependent on it should be removed once containerless scrolling looks stable.
 - name: layout.scroll.root-frame-containers
   type: RelaxedAtomicBool
   value: false
   mirror: always
 
 # This pref is to be set by test code only.
 - name: layout.scrollbars.always-layerize-track