This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Through The Eyes Of Another Pony
Firewall is a Timelord. His TARDIS was his packet of cigarets. Unfortunately, the influence of the Anti-Magic stuff screwed up his regeneration turning him into a firebreathing unicorn. Also, his TARDIS stop being a TARDIS and is now just a pack of cigarets.
The Mysterious Voice™ is the previous human to get dropped into Equestria.
The Mysterious Voice™ is a Timelord. He isn't the last human to show up in Equestria, as some suppose, but is a Timelord who got stuck there when DisQord stole his TARDIS. It certainly helps to explain his apparent semi-omniscience and longevity.
Discord is also Q, which means that My Little Pony: Frienship is Magic, Doctor Who and Star Trek exist in the same universe.
Also, Twilight Sparkle is clearly Luke Skywalker. I mean, why else would they play something that sounds an awful lot tike the music at the end of A New Hope when publicly recognizing her heroism, and she's already the most powerful thing in the known universe short of the local physical gods and is apprenticed to one of them. Therefore, the My Little Pony: Friendship Is Magic 'verse also includes Star Wars.
All of the above is an alternate universe within the Stargate Verse.
The whole thing is The Plan by the previous human to drop into Equestria.
Now proven to be not far from the truth.
The voice in Firewall's head is David Xanatos. All that has happend is part of his plan.
The voice in Firewall's head is the mirror opposite of the Nightmare. They were both created at the same time and will ultimately need to be merged back together.
The voice in Firewall's head is actually philomena. She's got the longevity to have been around 1000 years ago, and she's related to fire (Firewall's special talent) and thats why she can communicate with him.
Infecting Celestia with the Nightmare was always part of Luna's plan to bring about eternal night, since now she can legitimately hold the sun down and say it is for everypony's own safety.
The entire story can be reconciled with series cannon by taking place in the Equestrian timeline entirely after all of the events that have occurred and will ever occur through to the inevitable future end of the animated series; however, relative to Earth time, all such events have already occurred before the initial airing of the series' first season, thus Firewall would only have knowledge of the events that aired in the first season and would only learn of events from later seasons through some form of exposition from the other characters.
Firewall gets his phone "reformed" back sometime later. Then Celestia copies and absorbs it as well... including all the "Trollestia" and "Molestia" content. Which gives her bad ideas.
Firewall's body is part of the Nightmare; it was created to be the permanent vessel for the Nightmare. However, it didn't merge with it, but kept a connection. That's why it was able to bring Firewall into Equestria as a pony as opposed to a human. Also, what do you call a horse with flame powers and hair? A nightmare.
Now confirmed to be true.
Firewall is telling Part One of the story to someone on Earth. His references and asides might be an indication of this.
This is not a fanfic, it actually happened to the Author, hence the higher quality and great detail.
The story ends with Firewall at the very least in a state of contact with both Earth and Equestria. His memoirs are clearly written with the intent of being told to other bronies, but he also occassionally retracts statements 'at the insistence of a certain annoying pony...'
Everyone is Jesus in purgatory
David is in fact a future alternate-universe version of Notch, who having acquired the wealth of entire galaxies, invested in the development of the Codex, leading to the penultimate form of Minecraft that is now running a replica map of Equestria.
Celestia is going to give the Codex back to David in return for bringing back Firewall, because Luna is heartbroken without him. David does so, but uses this as a pact on Celestia and/or leaves out Firewall's memory (given the ending of chapter 14) so he won't prove to be much of a problem.