This Perk still needs more work. Pretty sure it's completely bugged now. Been playing it a lot as L25 on Hell on Earth.
If you don't have either Barbecue or Napalm, DoTs don't seem to work. At all. On the other hand, if you have either one of those Skills active, your DoT and splash damage is through the roof, currently making Firebug arguably one of the best trash cleaners. But only because something is broken: if you spray one target with fire, the Zeds around it will take massive damage and almost instantly melt. Even large Zeds take major damage. I think Napalm actually explosively increases the DoT damage, not the duration as the description would imply—or because the fire damage "bounces" too much in between Zeds, though I'm not sure about that yet. Because a few bursts of even the Caulk 'n Burn with Napalm active will clear out entire hordes of trash.
This makes taking Ground Fire and Shrapnel a terrible choice. Your DPS will be abysmal and you'll have trouble holding a line in Suicidal/Hell on Earth. Shrapnel's 20% chance of explosions does not hold up to the amount of damage Napalm will currently cause 100% of the time.
An excellent choice seems to be Ground Fire, Napalm, and Heat Wave. It gives the Firebug all the tools to play a competent defender for the team by melting a lot of Zeds and giving you a trick to keep Zeds off of teammates.
So it's very difficult to judge if the Firebug is better now, because it's so buggy. On one hand, it feels more like it should now, with Firebug being a competent trash cleaner. On the other hand, I feel like this is so broken that any fix will nerf the Firebug so badly that nobody will appreciate having the Perk on high difficulties again.
Ironically, its current broken state would be the right direction for it to go in because it's finally a good choice for trash cleaning right now in HoE. That said, I think that some of the future tweaks for Firebug should go into making the Perk tankier because its only defensive abilities are a passive fire damage reduction skill (against one single enemy type) and on L20 either Heat Wave or Firestorm are the only active skills that are useful in a defensive sense, but that's just me. Firestorm exposes you less to medium Zeds like Sirens, Bloats, and Husks, while Heat Wave is what's good to hold a line or get Zeds off of team mates. That's it—you still have to facetank a lot of damage, get no extra HP or armor, no regeneration, no damage reductions aside from Husk fire, and you don't even have any speed boosts that help you run faster. This is painfully noticeable whenever you're on a team where the Medic focuses heals on everybody else but you as a Firebug because of the Perk's notoriety as a bad trash cleaner, despite it currently being fairly easy to carry a wave solo even though I can sometimes watch the same team die get wiped out as soon as I die.
The muchly maligned Microwave Gun seems to be working as it should. I think most people don't understand how to use it effectively—set the Zeds on fire first with one two blasts of a CnB, then give them a short spray with the MWG, then alt-fire into them all lined up and that will clear a horde even including medium Zeds. That lets you conserve its ammo at maximum efficiency for whenever you need to push big Zeds away from the team to buy Demos/Sharps some time.
I'd like to see Flaratov's effects return as a passive skill now that it's gone. Especially if the Firebug gets fixed/nerfed from its current state, it would help tremendously.
One way I could imagine the Perk's skillset going is making decisions in between beefing up overall DPS versus defense/durability/debuffs. The whole concept of DoTs kind of needs to go. I like how the DoT skills increase the DoT damage instead of duration, this needs to stay that way. People don't appreciate it nor do they appreciate having a trash cleaner that can't kill things quickly enough.
Even if the current Perk skills were working correctly, the skill tree is not set up well, IMHO.
LV5: Choose between a skill that's only good to open with and then switch to the obvious choice; boosting your damage. I in fact don't even bother with the extra ammo and do fine on HoE.
LV10: Extra damage versus a debuff. A decent choice.
LV15: Damage versus damage. Kind of redundant. Don't 100% understand how the explosions fit thematically, anyway.
LV20: Defense versus defense. One is arguably more self-defense oriented (Firestorm) as it gives you more range, the other one is more team-defense oriented (Heat Wave) because it allows you to get Zeds out of people's faces or clear escape routes.
LV25: High DPS for no ammo use versus a strong debuff. Problem with the latter is that if you don't have extra range from Firestorm, or you have Ground Fire for the reliable speed debuff, it's often redundant. So not much of a choice here.