Can you provide an example public download link? Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?

Did you check the content of this file in a file viewer utility in case you get an HTML error file and not the actual file? Start Firefox in <b>Troubleshoot Mode</b> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if <b>hardware acceleration</b> or if <b>userChrome.css/userContent.css</b> is causing the problem. *switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes *do NOT click the "Refresh Firefox" button on 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 Boot the computer into Windows Safe Mode with network support to see if that has effect. *[https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/] *[https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/]