Add a more clear disclaimer for users who may be downloading harmful content

[claudijd]

Below is an example of what a user sees when they download a file. However, I think we should be more clear about the fact that this file is not "from Mozilla" and that there should be some sort of guidance/warning of some sort to educate users about malicious/harmful content that Mozilla/Firefox do not control as well as better guidance, tooling, and process for abuse claims.

[fzzzy]

I absolutely agree we should do this.

[claudijd]

Note that part of this is interrelated to #1479

[dannycoates]

started here 9891d1f