I loved my Firefly from the first day I got it, but sadly it just wasn't quite good enough, and it was ugly. But it shot fast and it shot pretty hard, and really that's all I wanted for my workplace war, distance doesn't matter a whole lot when you're in a cubical habitat.
So when I had some time on my hands, I decided to have a little fun with my Firefly.

Here's the stock FireFly.
Step 1: Remove all screws from the case, make note of the 3 screws that are different front the others, they go in the stock. You do NOT need to remove the two screws in the cocking lever, that will come out all on its own without removing the screws.
There should be a total of 20 screws including the one on the battery cover.

The firefly all taken apart, the ammo holders slide right out of the shell, and you can discard those goofy little connector bits, they don't really have a point in the gun except to make it look a little more strange.

Here's your internal view of the FireFly
Step 2: Remove all screws attached to the firing mechanism and cocking mechanism and remove the whole mess
The cocking lever can slide right off of the plunger assembly and be set aside. Make sure you note where all the screws go, there are several different types as well as the spring for the trigger. The orientation of the white plate above the trigger also matters a great deal because it controls the advancing of the barrels.

Here's the firefly gutless except for the strobe bulb and battery assembly. If you're going to be painting, or if you hate the whole strobe light idea, then do this next step, if not then skip it.
Step 4 (Optional): Remove all screws attached to the strobe light assembly and associated electronics. Make sure that the LED retainer bar and the on-off switch stay with the electronics package when you remove it.

Here's the strobe light assembly out of the firefly, note the 2 orange pieces, the LED retention bar is on the lower left and the on-off switch is on the lower right.

Here's your completely disassembled FireFly, be careful not to lose the front of the firing assembly, it's on a small spring and has a tendency to fly off at unexpected times.
If you're going to be replacing or adding on to the spring you can do it easily by removing the screw from the plunger head and placing a new spring on the plunger rod. I wouldn't do this personally being as the cocking mechanism is kind of flimsy, and you would definitely have to add a spring to the retention tooth so that the gun wouldn't randomly go off after being cocked.

Here's the triggering assembly and the turret, obviously the turret has been previously modified being as there are Stefans in there, but as you'll see in a bit, the mod process there is simple and doesn't need pictures.

Step 5: Remove the screw from the rear of the turret and remove the turret advancing mechanism.
Make sure you note the orientation of the retaining bracket, if you put it in backwards it won't work when you reassemble the gun.
Also be careful of the small toothed piece that resides under the mechanism, it likes to go flying.

Step 6: Remove 3 screws from the back of the turret that were uncovered by the removal of the advancing mechanism.

Step 7: Remove 3 screws from the front of the turret.

Step 8: Pry off the back of the turret with a screwdriver or something similar.
This is a pain in the tuckus. You're going to get annoyed with this thing, and the fit on the back of the barrels is so tight that it's always going to be a pain to remove to matter what you do. Fortunately replacing the barrels solves this issue.
Step 9: Remove the white air restrictors from the barrels. Just pry em out and chuck em in the bin, you don't need to keep the little white rings like is pictured here.

Here's your fully disassembled turret, at this point if you're not replacing the barrels with PETG or PVC then you can put everything back together and you're all done, if you're doing a barrel mod, read on.

Step 10: Cut off the stock barrels just in front of where they widen out at the front of the turret, then trim the edges until your barrel material of choice fits through them.

Here you can see the PETG in place with the front of the turret used as a guide.
Step 11: Glue the barrels in place on the back of the turret, you can use epoxy or super-glue or hot glue.
Myself I used Zap-A-Gap first and then after that dried, put hot glue over th make sure there were no air leaks. Do be careful though, the tip of a 40W hot glue gun CAN melt PETG.

Here's the completed turret with the outer clear-shell replaced, you can leave that bit off if you like, I prefer it on, just for the look of the thing, especially since I will be painting the Turret.

You can see the barrels and a bit of the hot glue in here, make sure you trim off any excess PETG from the front of the turret, there is almost no clearance between the front retaining clip and the front of the turret, any protruding PETG will cause the turret to fail to advance.
You're all set from this point if you're not painting, go ahead and reverse the disassembly process and you have a shiny new FireFly with PETG barrels.

Here's your Firefly reassembled, the placing of the white pieces is critical for the advancing mechanism to work correctly, so make sure that the turret advances before reassembling the shell.
Also make sure that the cocking lever is oriented correctly or the gun will never catch when you cock it.
Firefly Painting

Here's the shell of the FireFly laid out on a cardboard painting backdrop ready for priming.

Here's the first coat of gloss black vinyl dye, I'll do 2-3 more of these before I start with the primary colors.

Here's the firefly painted in its secondary color, I'll be making the gun silver and metallic red, somewhat like I did with the Iron Man 425 gun, but with more silver accenting. The taping on this one is going to be more intricate and much more difficult than the taping on the magstrike.
I also went over the turret with the special metallic flake clear coat to try and mitigate the effects of super-glue fumes on clear nerf plastic (Read: Cloudy instead of clear).

Here's the FF shell all taped and ready to be painted in red, the taping was a gigantic pain in the tuckus, but I got it done after only procrastinating for a month or so.

Here are the little bits and pieces, they're staying silver except for the battery cover in the lower right.

Here you have the firefly with the red coat all done and the tape still on, like the Iron Man 425 it looks pretty slick like this, but it will look even better with the tape off.

Another view, more clear in how the tape affected the colors, the tape actually ends up a rather interesting shade of deep purple, but that's for another gun.

Here's the gun all put together after the tape's been removed. As you can see I chose not to paint the ammo holders on the back and I chose to forgo the clear/sparkly turret in favor of a matching silver. I really like the effect of the silver turret much better than the clear, and since I made the turret a solid color, I also left out the strobe light which is why you can see two holes in the case, I'll fill them in later.

A view from the back, I really like how the stock turned out and how the area under the ammo holders looks, I think it struck just the right balance between dark and light.

A view from the front, and you can see that I missed painting the interior grills of the gun, I will remedy that once I get another can of red metallic paint, I ran out just as I was finishing up the battery cover on this gun. Luckily that was after I went over the Titan a couple times.
I like how this gun turned out, it looks a little bit like something Iron Man would have used in his Silver/Red armor phase, and a little bit like something you'd see in Gears of War or Team Fortress 2.
Distances when shot flat, 2 sets of 8 shots (Rounded down to the nearest foot):
42, 34, 34, 33, 33, 32, 32, 29
43, 40, 38, 36, 32, 32, 30, 29
I hope you enjoyed my Firefly modification guide, it's an easy mod, but it's still fun to do.
Suggestions and constructive criticisms are always welcome, as well as questions.
You can find my Flickr based guides and a bunch of pictures here:
Nerf Pictures and Writeups