It's adding a debian-compatible repo (from Linux Mint Debian Edition) in order to replace Iceweasel (firefox without the branding) with Firefox (firefox with the branding). Firefox's branding is incompatible with Debian's licensing requirements, and they amicably split the branding years ago as a result.

The example was intentionally selected to be as obfuscatory as possible, in order to be a troll. Most of those commands are about adding a repository rather than installing software, anyway (and the four lines involving gpg are usually done in a single line)