From https://www.reddit.com/r/firefox/comments/jgim0d/beta_830b3_screen_flickering/g9qo22d/

I watch a twitch stream on one monitor and browsing reddit on the main one, main window starts to flicker

Main display is 2560x1440@165Hz and secondary displays are 2560x1440@59Hz

Disabling WebRender resolves the issue.

Similar issue reported:

I can confirm this behavior & I'm also on 83.0b3 currently. I have a dual monitor setup and the issue is reproducible if I have a video (YouTube) playing in a window on one monitor and then mouse over various screen elements in another window on the other monitor. Sometimes you have to scroll the page a bit for it to start again, as it seems to "calm down" after while. For me it works most consistently with twitter embeds (that themselves embed images) on a forum I'm frequenting, but also to a lesser extent on twitter itself. If the video player size is reduced beyond a certain point the glitches go away. They also disappear as soon as no part of the video playback happens on the second monitor.

about:support following this comment.

Sounds like bug 1638709

The severity field is not set for this bug.
:jimm, could you have a look please?

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