Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click "Refresh Firefox" in the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I have switched to troubleshoot mode and the problem was still there.

I don't know about hardware acceleration.

The website doesn't use userChrome.css if that is what you mean by that.

It seems to happen to all websites, some using different software, on a particular server: Intel(R) Xeon(R) E-2286G CPU @ 4.00GHz Intel64 Family 6 Model 158 Stepping 10 ~4008MHz Plesk Obsidian v18.0.65_build20241122.08 os_Windows 2012/2016/2019/2022 Microsoft Windows Server 2022

I have another server running sites with the same software and version and they seem to be fine: Intel(R) Xeon(R) CPU E5-1650 v3 @ 3.50GHz Intel64 Family 6 Model 63 Stepping 2 ~3500MHz Plesk Obsidian v18.0.64_build20241008.15 os_Windows 2012/2016/2019/2022 Microsoft Windows Server 2019

You can create a new profile as a quick test to see if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

If the new profile works, you can transfer files from a previously used profile to the new profile, but be careful not to copy corrupted files to avoid carrying over problems.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

We tried both new profile and safe mode but neither solved the problem.

We have noticed that sometime stylesheets and scripts don't load when we switch and come back. However, we can't replicate that. Perhaps its on first load of a site and once the styles and scripts are cached it's not a problem.

Any other suggestions would be greatly appreciated.

I can confirm that when I go to a site for the first time, I picked a page that would take a while to load, and switch tabs before the HTML arrives, it does not load the styles, scripts, etc. I was able to repeat it over and over. When I stayed on the tab and waited for the load it worked and I could no longer repeat because of the cache I presume.

Any other suggestions to try?