Okay, here is what I think of the new FB tree as it currently stands.
EDIT: I've updated a couple bits just to add a suggestion or two I think would be cool.
L5 - both good choices, hard to pick. Straight up double ammo or more damage, tough choice! How tough the choice is depends entirely on what that 35% damage applies to. If that applies to impact, splash and afterburn (all fire basically) then it's a great start and tough choices.
L10 - Again decent choices here, BBQ duration sounds fun but it only actually sounds impactful if the "Bring the Heat" damage bonus applies to it. Ground fire is cool, I never tend to play that way though so I'm focused on the duration here. The slowing effect makes this a tough choice, though like I say I'm not one to shoot the floors much.
my suggestion: Duration should be given to groundfire and the slow effect be given to either BBQ or napalm. The reasoning is the slow doesn't need to be paired with the extra damage whereas a longer DoT would greatly benefit from a slow.
L15 - There's a bit of a problem here. This looks pretty much the same as it did before. Shrapnel was Pyre is now Firebomb - I'm assuming this is balanced so no worries there. Napalm sounds the same though, duration or not. Let me put it this way - If anyone picks Napalm, I can promise you it is solely for duration, which in turn isn't good enough.
My one main wish with FB as a perk was to be able to choose the other level 15 skill - no firebombs or shrapnels just a burny dancy time. Napalm however, even if it flawlessly spread fire to whole hordes alone, only does the basic afterburn rate. Napalm needs to do something else to keep up, maybe strengthened panic or a vulnerability debuff; Something which feels like you've just upgraded to top-shelf nightmare ammo. In my opinion how Napalm is handled is the crux of how good this remake will be.
My suggestion: Napalm, whether it can spread fire or not, should have something to work alongside the longer burn time. Again, the slow effect from groundfire would be quite good here and also fitting of the name "Napalm". My other thought on it was perhaps either a 10% vulnerability to on-fire enemies or an increase in panic power/duration. That last one would work nice with a slow effect!
L20 - Nice choices and a good idea, get stuck in or try and keep back. Good choices, range should probably have a little something to compete with heatwave however. Maybe a much less frequent or lasting stumble from further ranges or on ticks of fire? Just spitballing. Otherwise good choices, good idea. Happy to see the ugly splash damage gone!
My suggestion: Heatwave is a great tool for keeping stuff away from you and others but range alone can't really compete with that utility, considering a large portion of fighting is closer to heatwave-ranges. Range is nice but it needs something to keep them back at that range a bit more for it to be useful. Though, if BBQ or Napalm applied a slowing effect instead of Ground Fire I can see range being an equal to Heatwave!
L25 - It's no secret I am not a fan of the final skills being basically RNG but I can deal with these. Both sound nice, the vulnerability/damage one sounds like it'd be short-lived and not have too much application in ZT, unless it includes everything already on fire and not just what you're shooting. If that is the case then yes, these are 2 good choices (for ZT skills anyway). The infinite ammo one sounds fun to be fair.
That's my bit! In summary: Most of it is good, duration sounds okay but fire needs to do more I think, and the core of this being a good rework is making Napalm a stronger skill. Zeds on-contact spreading fire is not good enough to contend with Firebomb I'd say, and needs a stronger effect than that to stay relevant - And really, one of the only things I want out of firebug is the non-shrapnel skill to be relevant.