--- a/gfx/thebes/gfxPlatformGtk.cpp
+++ b/gfx/thebes/gfxPlatformGtk.cpp
@@ -635,16 +635,17 @@ public:
   }
 
   class GLXDisplay final : public VsyncSource::Display
   {
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(GLXDisplay)
 
   public:
     GLXDisplay() : mGLContext(nullptr)
+                 , mXDisplay(nullptr)
                  , mSetupLock("GLXVsyncSetupLock")
                  , mVsyncThread("GLXVsyncThread")
                  , mVsyncTask(nullptr)
                  , mVsyncEnabledLock("GLXVsyncEnabledLock")
                  , mVsyncEnabled(false)
     {
     }