diff --git a/widget/nsBaseAppShell.cpp b/widget/nsBaseAppShell.cpp
index aa6468554b9..be813c6b735 100644
--- a/widget/nsBaseAppShell.cpp
+++ b/widget/nsBaseAppShell.cpp
@@ -31,6 +31,7 @@ nsBaseAppShell::nsBaseAppShell()
   , mSwitchTime(0)
   , mLastNativeEventTime(0)
   , mEventloopNestingState(eEventloopNone)
+  , mRunningSyncSections(false)
   , mRunning(false)
   , mExiting(false)
   , mBlockNativeEvent(false)
@@ -350,6 +351,11 @@ nsBaseAppShell::RunSyncSectionsInternal(bool aStable,
   NS_ASSERTION(NS_IsMainThread(), "Wrong thread!");
   NS_ASSERTION(!mSyncSections.IsEmpty(), "Nothing to do!");
 
+  // We don't support re-entering sync sections. This effectively means that
+  // sync sections may not spin the event loop.
+  MOZ_RELEASE_ASSERT(!mRunningSyncSections);
+  mRunningSyncSections = true;
+
   // We've got synchronous sections. Run all of them that are are awaiting a
   // stable state if aStable is true (i.e. we really are in a stable state).
   // Also run the synchronous sections that are simply waiting for the right
@@ -377,6 +383,7 @@ nsBaseAppShell::RunSyncSectionsInternal(bool aStable,
   }
 
   mSyncSections.SwapElements(pendingSyncSections);
+  mRunningSyncSections = false;
 }
 
 void
diff --git a/widget/nsBaseAppShell.h b/widget/nsBaseAppShell.h
index b2b9479ecbf..b528e88e484 100644
--- a/widget/nsBaseAppShell.h
+++ b/widget/nsBaseAppShell.h
@@ -134,6 +134,7 @@ private:
   };
   EventloopNestingState mEventloopNestingState;
   nsTArray<SyncSection> mSyncSections;
+  bool mRunningSyncSections;
   bool mRunning;
   bool mExiting;
   /**