STR:

1. Collect some amount of items in Downloads
2. Scroll it with mouse scroll or with scroll button
   and enjoy flickers/blinks/flashes on scrolling

No issue with older native Downloads GUI.

:Virtual, could you try to find a regression range in using for example mozregression?

I don't know it it will be useful but I have taken profiles with Firefox Profiler

• Firefox Profiler profile with bug (new Downloads GUI) - https://perfht.ml/2O010Xk
• Firefox Profiler profile without bug (old Downloads GUI) - https://perfht.ml/2O02C3k

It's probably caused by bug #675902, but mozregression-gui fails me when searching for very old builds, so it's only my suspicion on based on many bookmarked Bugzilla bugs by me.

the list of downloads is populated lazily, so it's possible it will show a white list while loading more entries. The alternative would be UI hangs. We could/should maybe preload a small portion in addition to the visible one. That could reduce the problem.

Though, how did you generate such a long list? By default we only retain downloads for 60 days, so it's unlikely to happen.

Alternative "old" UI works flawlessly without any issues, as you can see on my screencast, so it's clearly problem with "new" UI, which isn't that much performant, as "old" UI.

I remember same issue with new UI for about:config. I reported it in bug #1524078, which was duped to bug #1523028. Please see also blockers and dependencies of bug #1523028 for more information what could be cause of it.

I'm downloading galleries with pictures using DownThemAll! alternative which is Download Star extension.
But in these 60 days (new to me and good to know specific purge live time limit of download items) I can easily download by hand manually over few thousands of files.
Either way, issue isn't how I use browser, but about:downloads performance and that it's less performant compared to how it was before in past. ;)

I also don't have this issue using UnGoogled-Chromium in comparative tests.