How did you download the ISO, direct or as a torrent? Have you checked the properties of the file, is it the gigabytes the download page says it should be (2.4 gb)? Have you checked the MD5 hash? GtkHash works good for that. Have you tried something other than Rufus, like Etcher on Linux or LILI (LinuxLive USB Creator) on Windows? The Firefox Icon is a clue, I don't think Rufus or the others will find it if it is not an ISO. It should look like this "ubuntu-mate-20.04.1-desktop-amd64.iso"