--- a/gfx/thebes/gfxWindowsPlatform.cpp
+++ b/gfx/thebes/gfxWindowsPlatform.cpp
@@ -2896,16 +2896,21 @@ public:
 
         DWM_TIMING_INFO vblankTime;
         // Make sure to init the cbSize, otherwise GetCompositionTiming will fail
         vblankTime.cbSize = sizeof(DWM_TIMING_INFO);
 
         LARGE_INTEGER frequency;
         QueryPerformanceFrequency(&frequency);
         TimeStamp vsync = TimeStamp::Now();
+        // On Windows 10, DwmGetCompositionInfo returns the upcoming vsync.
+        // See GetAdjustedVsyncTimestamp.
+        // On start, set mPrevVsync to the "next" vsync
+        // So we'll use this timestamp on the 2nd loop iteration.
+        mPrevVsync = vsync + mSoftwareVsyncRate;
 
         for (;;) {
           { // scope lock
             MonitorAutoLock lock(mVsyncEnabledLock);
             if (!mVsyncEnabled) return;
           }
 
           // Large parts of gecko assume that the refresh driver timestamp