I got the screenshot on https://www.youtube.com/watch?v=qyCCGIp-uZc when set to 1080p fullscreen on Developer Edition 74 without WebRender, but have noticed it for a while…
I'll leave further details as I notice it again.

Just seen again at https://share.getcloudapp.com/GGuNpgLn on DevEdition 75.0b1 (64-bit).
Looks like the bottom of a previous frame, not the top of any of the preceding frames.
Also, it's not animating, it's just a static row of pixels.
Scrolling up and down seems to re-set it.

And in https://share.getcloudapp.com/DOu8BZ9K you can see that it's not the whole line, just part of it. (The previous shot was mostly white.)

Blake, is this something you can reproduce reliably with that version of Firefox?

Could you attach the output from about:support?

Yes, and I've seen it for a few versions now. (Sorry for not reporting it sooner.)

Hmm, I could not reproduce the problem on my Mac.

:mstange, do you know if there is a similar bug?

I can reproduce this when I switch to the default "scaled" resolution on my machine, which is 1680x1050@2x.

I cannot reproduce this on a build that contains the patches for bug 1595038. I'll ask Blake to re-test once those patches land.

Blake, can you still reproduce this? The fix for bug 1595038 is in now in Firefox Nightly.

I wasn't able to in the 8 minutes I tested, and could immediately reproduce it on Developer Edition, so it looks like this is fixed in Nightly! 🎉

Because this bug's Severity has not been changed from the default since it was filed, and it's Priority is P3 (Backlog,) indicating it has been triaged, the bug's Severity is being updated to S3 (normal.)