https://www.bleepingcomputer.com/news/microsoft/some-windows-10-users-cant-use-firefox-for-downloads/

Mozilla says in an update to the Firefox 69.0.2 changelog that downloading files from the Internet may not work for Windows 10 users who have the Parental Controls feature toggled on and are part of a family group.

The problem is described on Reddit [1, 2, 3] and Mozilla's bug tracker in an issue reported three days ago, on October 4, saying that all web downloads are failing when using Firefox on Windows 10.