When the screen pops up hit the print screen button and post it.

Not sure that's possible. It happens too fast. Doing a refresh now, if that doesn't work I'll uninstall and reinstall.

Reinstalled twice and still the same issue.

Have you tried safe mode?

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Do you have the same issue using a different windows user account?

Safe mode does not have the problem. Only one user account, I'll have to set up another one. The only add-on is UBlock, ad blocker, and disabling it makes no difference. This is only happening on a Win 7 desktop. Win 10 laptop has no problems.

When uninstalling and then reinstalling a window comes up informing me that Firefox had been previously installed and that settings and favorites will be restored or something to that effect. Is it possible the bug is being restored? Is there a way to have the system forget that Firefox was ever installed?

Does it happen only on startup, or also on invoking the Home page during browsing?

Only happens on startup.

It might be a pre-xul skeleton. It's a visual indicator that something is being loaded in background on slower systems. If you want to turn it off, see https://www.ghacks.net/2021/01/25/firefox-nightly-uses-a-new-skeleton-ui-on-start-on-windows/

No change. Followed the instructions, set skeleton to off. Rebooted, still shows up. It is a very slow computer, so really no big deal. Just curious.

I tested this on Linux as I see a short blank (black) window as well on startup, especially on Nightly. On Linux the browser.startup.preXulSkeletonUI isn't available and using backspace to see what prefs would show up came up with the blankWindow pref. Setting this pref to false made Nightly start without showing the black window.

• browser.startup.blankWindow = false