Hi Elin

What happens when you try to open an .apk from that site?

I would always recommend either installing apps directly from Google Play or (if you trust the developer) side loading directly from their own code repository.

When downloading files like an APK, Firefox might sometimes mislabel the file type. The file is probably fine, but it just needs its extension changed.

Solution Find the downloaded file on your computer. It might be in your "Downloads" folder.

Rename the file so that it ends with .apk. For example, if it's named appfile, change it to appfile.apk.

After you've changed the name, try opening the file again to install it. It should work properly now.