A creator might name a test file "fff" quickly.

If you’ve stumbled upon a file named on your computer or in a download folder, you might be wondering what it is and if it's safe to open. While the name "fff" is likely just a label given by the creator, the .7z extension tells us a lot about the file's technology. What is a .7z File?

Use free utilities like The Unarchiver or Keka to extract the files.

Below is a draft for a blog post explaining what this file is, how to open it, and why this format is used. Demystifying fff.7z: What It Is and How to Open It