Question macbook pro i9 16". horizontal red flash lines atop firefox on mouse or keyboard sometimes

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Apr 27, 2021
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This is catalina. 64 gigs of ram. i9 8-core intel. 16 inch. Only with firefox sometimes there are shifting, intermittent, red dashed lines along the very top edge of the firefox app border. This is the AMD Radeon Pro 5500M 8gig. It's basically impossible to screenshot. :(

What causes this? how can I make it stop. It's super distracting. It happens on firefox windows that are not in the foreground also.
 
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