Firefox should not modify the actual contents of a downloaded file. However, it sometimes does change the file extension causing programs to be unable to open them. If you set Windows to show all file extensions, can you tell whether Firefox is changing them in an inappropriate way? This article has the steps if needed:

https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

By the way, your Firefox has a funny bit at the end of the "user agent" string it sent to this site. Any idea what that might indicate?

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:97.0) Gecko/20100101 Firefox/97.0 Config/94.2.9111.12

By the way, there is a setting to reduce the file extension renaming problem so we could set that to the side. It's a bit obscure how you switch it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste browser.download.sanitize_non_media_extensions and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

Let me know whether you notice any difference on future downloads.

Maybe also check your security software as they might be inspecting the archive and possibly corrupt it.

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Boot the computer in Windows Safe Mode with network support to see if that has effect.

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

Win11's Edge downloads .zip files just fine. However I'm still having this error using firefox even after changing the config browser.download.sanitize_non_media_extensions to false. It's not AVG either, since Edge works fine.

So the problem is that you download a ZIP file, and Windows or your unzip program says it is not a zip file?

If you compare the files downloaded in Firefox and Edge, do you notice major differences, for example, is the file downloaded in Firefox much smaller? I mention that because sometimes a site will replace a file with an error page, but Firefox won't recognize the switch and saves it under the original name. You can change the file extension to .txt (text file) so it will open in a text editor, and then check its contents and see whether it looks like an HTML page. (For safety, I wouldn't rename it with a .html extension until you know exactly what is in it.)