Mozilla issues issue in Firefox 69.0.2 and 70.0b11+ that causes downloads to fail in Windows 10 when you have parental controls or content restrictions on. The problem is described on Reddit [1, 2, 3] and Mozilla’s bug tracker in an issue reported three days ago. A patched version is not yet available in the Stable or Beta channels, the releases affected by this issue are not yet released. Users who have already installed the impacted Firefox versions will have to downgrade to a previous stable version.
Source: https://www.bleepingcomputer.com/news/microsoft/some-windows-10-users-cant-use-firefox-for-downloads/