Firebug was the manager of the Pebble district heating plant. She disappeared from Equestria with the Arcane Helix while trying to rescue some comrades from it, thereby arriving in the Dread Dimensions. Firebug lost her unicorn magic to a strange illness after being attacked by a monster in the Helix; she got it back (and then some) by becoming a priestess of the goddess En-Bequ'avar, Shadhavar deity of fire, forge and hearth. Firebug participated in the defeat of unicorn lich Dusty Tomes. She was made the Matriarch of the Outer Lands for political reasons, thereby being promoted to the status of archpriestess only a few days after her original ordination. En-Bequ'avar, via Firebug, killed the dragon Scört in the Holy Maw of the Slimefather. Firebug returned to Equestria, where she redeveloped the derelict Iron Tower of Neighswango and lived for a very long time thereafter thanks to magical life extension.

Firebug was romantically involved with the pegasus Shady Jay. She was good friends with Wiggles and Epona Moonbeam. Her relationship with Revolution Rosie was highly variable.

Left to right: Firebug at the Pebble district heating plant, Firebug wearing Shear Steel's robe and diadem, Firebug as the Matriarch of the Outer Lands (later, of Equus) of the Temple of En-Bequ'avar. Note that the artist's conception on the far right does not depict her usual cutie mark.

Quotes

Nate‬: Shall I throw him out, ma'am?
10:45 PM
‪Bryce‬: Firebug: ... that won't be necessary. One of the ancient sages said, "don't rip the limbs off the messenger and incinerate his sundered corpse."

‪me‬: [unaware that Firebug is in a murderous rage]
10:32 PM
‪Bryce‬: (You wouldn't necessarily know if she is. When enraged, Firebug prefers to let others do the screaming.)

The infamous "invention of murder" conversation:

Aug 07 23:04:29 <Firebug>    (So Firebug and Rosie are alone now, right?)
Aug 07 23:04:58 <Owenmighty>    Yes.
Aug 07 23:05:26 <Firebug>    Firebug: Do you ever wonder who invented murder, Rosie?
Aug 07 23:05:41 <Eppina>    (oohhhhhhhhh dear.)
Aug 07 23:05:47 <Wyjolz>    (AWWW shit.)
Aug 07 23:05:50 <Rosie>    Rosie: Not you, if you're boasting.
Aug 07 23:05:51 <Wyjolz>    (this is awesome)
Aug 07 23:05:56 <Wyjolz>    (creepy Firebug)
Aug 07 23:06:10 <Firebug>    Firebug: Oh, of course not. But, don't you wonder?
Aug 07 23:06:12 <J>    (wow, that line of questioning got dark all of the sudden)
Aug 07 23:06:47 <Rosie>    Rosie: I assume a bad pony, long ago.
Aug 07 23:09:11 <Firebug>    Firebug: Oh? I don't think so. I think it was a very good and noble pony who invented murder.
Aug 07 23:09:37 <Firebug>    Before the invention of murder, after all, it was just killing.
Aug 07 23:09:48 <Rosie>    Rosie: That's insane! You're insane.
Aug 07 23:09:57 <Wyjolz>    (That is good justification to kindly and nobly stab clients tomorrow when their whining and stupidity pisses me off.)
Aug 07 23:10:16 <Firebug>    But some pony decided that certain killing was murder. A crime. Evil.
Aug 07 23:10:18 <Owenmighty>    (Easy now, Lauren.  You can't set the police on fire with your mind, after all.  :D)
Aug 07 23:10:27 <Wyjolz>    (I wish I could.)
Aug 07 23:10:43 <Wyjolz>    (I am the firestarter. I am the bitch you hated. RAAAAARGH!)
Aug 07 23:11:22 <Firebug>    Just like some pony decided which side of the street we drive carts on, and what day of the week the streets will be swept.
Aug 07 23:11:43 <Rosie>    Rosie: Fancy words don't change how killing ponies is wrong.
Aug 07 23:11:59 <Firebug>    Oh? So it's always wrong? Killing itself?
Aug 07 23:12:30 <J>    (wow, Firebug's kinda maniacal, but has a valid point)
Aug 07 23:12:40 <Rosie>    Rosie: It is, even when excused in some way, at best it will only be a lesser evil.
Aug 07 23:13:09 <Wyjolz>    (Wow, GO, BRYCE, GO! This is fucking awesome.0
Aug 07 23:13:14 <Firebug>    That's surprisingly insightful. Yes, indeed. Everything is always only a lesser or greater evil.
Aug 07 23:13:17 <Eppina>    (good god)
Aug 07 23:13:40 <Owenmighty>    (Teambuilding!)
Aug 07 23:14:12 <Firebug>    And here I find myself, in a deadly forest with you, the apple of discord among us.
Aug 07 23:14:26 <Firebug>    We wouldn't even be in Neighswango if not for you.
Aug 07 23:14:36 <Rosie>    Rosie: Is this discussion headed somewhere. Veiled threats?
Aug 07 23:15:11 <Firebug>    [A bit more emotionally] I wouldn't have this diadem tearing into my soul if not for you.
Aug 07 23:15:49 <Firebug>    (You know, Jay, it might be a good idea to show up soon)
Aug 07 23:16:19 <Rosie>    Rosie: Oh, so you were lying that it wasn't harmful to our souls?
Aug 07 23:16:26 <Owenmighty>    (Firebug killing Rosie could lead to Bad Things for the party, yeah.)
Aug 07 23:16:33 <Owenmighty>    (Though it _would_ be interesting.)
Aug 07 23:16:49 <Firebug>    I don't know enough about souls to be sure.
Aug 07 23:16:58 <Owenmighty>    Jon, are you here?
Aug 07 23:17:07 <Rosie>    Rosie: You put it in willingly. You decided Neighswango was an interesting place to recuperate. You took us into the Everfree Forest!
Aug 07 23:17:37 <Firebug>    It was going just fine until we had to look for a place to heal your leg, that was only injured because you ran headlong into that fight with a kelpie!
Aug 07 23:17:57 <Firebug>    Sabotage! You sabotaged us. Does Crunchy want the diadem?
Aug 07 23:18:03 <Firebug>    Is that why we're out here?
Aug 07 23:18:04 <J>    (yah, sry, everyone else just had such interesting things going on! XD)
Aug 07 23:18:18 <Owenmighty>    (Wow, Firebug is getting _paranoid_.)
Aug 07 23:18:35 <Rosie>    Rosie: And whose idea was it to head into a swamp with dangerous creatures with inexperienced heating plant workers?
Aug 07 23:19:01 <Firebug>    We handled the Kelpie just fine until you got yourself injured by it.
Aug 07 23:19:15 <Rosie>    Rosie: I know nothing of the mayor, other than that I don't like him. How should I know if he wants that accursed thing? Let him have it.
Aug 07 23:20:22 <Firebug>    Firebug:  hmm. 
Aug 07 23:21:22 <Firebug>    Firebug: Maybe I should just leave. He obviously has agents everywhere.
Aug 07 23:21:44 <Firebug>    I should have known this small-time politician thing was just a front.
Aug 07 23:21:45 <Wyjolz>    So...you're saying that one of us is the right end...and the other is the left?
Aug 07 23:21:50 <J>    (can he hear either Rosie's or Firebug's voice?)
Aug 07 23:22:03 <Owenmighty>    Hm...
Aug 07 23:22:13 <Rosie>    Rosie: You're delusional. Think about what that diadem is doing to you and us.
Aug 07 23:22:21 <Owenmighty>    ...Yes, he can tell that Rosie's up there.
Aug 07 23:22:48 <Firebug>    Firebug: It may be better if we don't return to Pebble.
Aug 07 23:23:36 <Rosie>    Rosie: Where would we go? Ponyville?
Aug 07 23:24:04 <Firebug>    Firebug: I certainly wouldn't tell you.
Aug 07 23:24:15 <Firebug>    There's no we here.