--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -87,46 +87,44 @@ using namespace mozilla::dom;
     RefPtr<type_> result;            \
     result.swap(mPtr);               \
     return result.forget();          \
   }
 #include "mozilla/ServoArcTypeList.h"
 SERVO_ARC_TYPE(StyleContext, ServoStyleContext)
 #undef SERVO_ARC_TYPE
 
-static Mutex* sServoFontMetricsLock = nullptr;
-static Mutex* sServoWidgetLock = nullptr;
-static RWLock* sServoLangFontPrefsLock = nullptr;
+static RWLock* sServoFFILock = nullptr;
 
 static
 const nsFont*
 ThreadSafeGetDefaultFontHelper(const nsPresContext* aPresContext,
                                nsIAtom* aLanguage, uint8_t aGenericId)
 {
   bool needsCache = false;
   const nsFont* retval;
 
   {
-    AutoReadLock guard(*sServoLangFontPrefsLock);
+    AutoReadLock guard(*sServoFFILock);
     retval = aPresContext->GetDefaultFont(aGenericId, aLanguage, &needsCache);
   }
   if (!needsCache) {
     return retval;
   }
   {
-    AutoWriteLock guard(*sServoLangFontPrefsLock);
+    AutoWriteLock guard(*sServoFFILock);
   retval = aPresContext->GetDefaultFont(aGenericId, aLanguage, nullptr);
   }
   return retval;
 }
 
 void
 AssertIsMainThreadOrServoLangFontPrefsCacheLocked()
 {
-  MOZ_ASSERT(NS_IsMainThread() || sServoLangFontPrefsLock->LockedForWritingByCurrentThread());
+  MOZ_ASSERT(NS_IsMainThread() || sServoFFILock->LockedForWritingByCurrentThread());
 }
 
 bool
 Gecko_IsInDocument(RawGeckoNodeBorrowed aNode)
 {
   return aNode->IsInComposedDoc();
 }
 
@@ -855,17 +853,17 @@ Gecko_IsDocumentBody(RawGeckoElementBorr
 
 nscolor
 Gecko_GetLookAndFeelSystemColor(int32_t aId,
                                 RawGeckoPresContextBorrowed aPresContext)
 {
   bool useStandinsForNativeColors = aPresContext && !aPresContext->IsChrome();
   nscolor result;
   LookAndFeel::ColorID colorId = static_cast<LookAndFeel::ColorID>(aId);
-  MutexAutoLock guard(*sServoWidgetLock);
+  AutoWriteLock guard(*sServoFFILock);
   LookAndFeel::GetColor(colorId, useStandinsForNativeColors, &result);
   return result;
 }
 
 bool
 Gecko_MatchStringArgPseudo(RawGeckoElementBorrowed aElement,
                            CSSPseudoClassType aType,
                            const char16_t* aIdent)
@@ -1299,17 +1297,17 @@ Gecko_nsFont_InitSystem(nsFont* aDest, i
   // itself, so this will do.
   nsFont* system = new (aDest) nsFont(*defaultVariableFont);
 
   MOZ_RELEASE_ASSERT(system);
 
   *aDest = *defaultVariableFont;
   LookAndFeel::FontID fontID = static_cast<LookAndFeel::FontID>(aFontId);
 
-  MutexAutoLock lock(*sServoFontMetricsLock);
+  AutoWriteLock guard(*sServoFFILock);
   nsRuleNode::ComputeSystemFont(aDest, fontID, aPresContext, defaultVariableFont);
 }
 
 void
 Gecko_nsFont_Destroy(nsFont* aDest)
 {
   aDest->~nsFont();
 }
@@ -2348,22 +2346,22 @@ Gecko_nsStyleFont_PrefillDefaultForGener
 void
 Gecko_nsStyleFont_FixupMinFontSize(nsStyleFont* aFont,
                                    RawGeckoPresContextBorrowed aPresContext)
 {
   nscoord minFontSize;
   bool needsCache = false;
 
   {
-    AutoReadLock guard(*sServoLangFontPrefsLock);
+    AutoReadLock guard(*sServoFFILock);
     minFontSize = aPresContext->MinFontSize(aFont->mLanguage, &needsCache);
   }
 
   if (needsCache) {
-    AutoWriteLock guard(*sServoLangFontPrefsLock);
+    AutoWriteLock guard(*sServoFFILock);
     minFontSize = aPresContext->MinFontSize(aFont->mLanguage, nullptr);
   }
 
   nsRuleNode::ApplyMinFontSize(aFont, aPresContext, minFontSize);
 }
 
 void
 FontSizePrefs::CopyFrom(const LangGroupFontPrefs& prefs)
@@ -2422,58 +2420,52 @@ void
 InitializeServo()
 {
   URLExtraData::InitDummy();
   Servo_Initialize(URLExtraData::Dummy());
 
   gUACacheReporter = new UACacheReporter();
   RegisterWeakMemoryReporter(gUACacheReporter);
 
-  sServoFontMetricsLock = new Mutex("Gecko_GetFontMetrics");
-  sServoWidgetLock = new Mutex("Servo::WidgetLock");
-  sServoLangFontPrefsLock = new RWLock("nsPresContext::GetDefaultFont");
+  sServoFFILock = new RWLock("Servo::FFILock");
 }
 
 void
 ShutdownServo()
 {
-  MOZ_ASSERT(sServoFontMetricsLock);
-  MOZ_ASSERT(sServoWidgetLock);
-  MOZ_ASSERT(sServoLangFontPrefsLock);
+  MOZ_ASSERT(sServoFFILock);
 
   UnregisterWeakMemoryReporter(gUACacheReporter);
   gUACacheReporter = nullptr;
 
-  delete sServoFontMetricsLock;
-  delete sServoWidgetLock;
-  delete sServoLangFontPrefsLock;
+  delete sServoFFILock;
   Servo_Shutdown();
 }
 
 namespace mozilla {
 
 void
 AssertIsMainThreadOrServoFontMetricsLocked()
 {
   if (!NS_IsMainThread()) {
-    MOZ_ASSERT(sServoFontMetricsLock);
-    sServoFontMetricsLock->AssertCurrentThreadOwns();
+    MOZ_ASSERT(sServoFFILock &&
+               sServoFFILock->LockedForWritingByCurrentThread());
   }
 }
 
 } // namespace mozilla
 
 GeckoFontMetrics
 Gecko_GetFontMetrics(RawGeckoPresContextBorrowed aPresContext,
                      bool aIsVertical,
                      const nsStyleFont* aFont,
                      nscoord aFontSize,
                      bool aUseUserFontSet)
 {
-  MutexAutoLock lock(*sServoFontMetricsLock);
+  AutoWriteLock guard(*sServoFFILock);
   GeckoFontMetrics ret;
 
   // Getting font metrics can require some main thread only work to be
   // done, such as work that needs to touch non-threadsafe refcounted
   // objects (like the DOM FontFace/FontFaceSet objects), network loads, etc.
   //
   // To handle this work, font code checks whether we are in a Servo traversal
   // and if so, appends PostTraversalTasks to the current ServoStyleSet