This is a follow up from bug 1543356 and bug 1546371 which is focused on Ubuntu. This issue was opened on request based on comment 13.

A reminder that we can use the steps from Bug 1543356 in order to verify this fix :

[Prerequisites]:

Ensure that in about:support at Graphics section “Compositing” has the “WebRender” attribute, if not set gfx.webrender.all to true.
Possible Nvidia GPU needed for enabling WebRender.
Steps to reproduce

Open Firefox and open at least 4 tabs.
Continuously drag the browser from the bottom area to resize it and move the mouse up and down.
Observe the tabs section.

Hi,
I've tested this using the latest Nightly version 74.0a1 (2020-01-13) (64-bit) for Ubuntu 18.04.3 LTS and I’m able to reproduce the issue. Based on this I will mark firefox74 flag as affected. I can reproduce A, B and C from the description. I'm attaching a new video showing that withing about:preference submenues these glitches are also present when resizing.
Best,
Clara

Clara, I can't reproduce this here. Does it still reproduce for you?

Hi,
This is what it currently looks like on 88.0a1 (2021-03-18) (64-bit)
Best,
Clara

Thanks Clara. I'm not entirely sure this is really fixable on X11 without changing our whole GTK architecture - X11s async nature makes things very difficult. On Wayland it's currently also not great but should be properly fixable. So this is likely a wontfix :/