Please update to use Firefox 60 (Either ESR or regular edition). I'd also suggest that you not use Avast, but just use Windows Defender (keep Windows 10 up to date, 1803 is the latest build). Then, update your graphics driver ([[Upgrade your graphics drivers to use hardware acceleration and WebGL]]). Finally, if you still have issues, refresh Firefox. [[Refresh Firefox - reset add-ons and settings]]