In-app Flashes can be enabled or disabled on your account by toggling the small bel icon in the top right-hand corner on your navigation bar. If In-App Flashes are enabled, the bel will show blue. Otherwise, it will appear faded out.
If you disable in-app flashes, you won't receive any notification for when an in-app flash goes off.
This setting can be easily toggled at any time by clicking the bel icon.