A little discussion in the chat with Isen, Arma and Zock lead me to dig again into this unit behaviour, and retrieve some optimizations we did with Brute51 in the Community Balance Patch.
These little guys need a little love. There are several problems plaguing them:
1) they are hard to use because of the range. It can only work efficiently against small targets because it has only range 0-3. And a factory is bigger than this. Which means that if you order it to attack a factory, it will walk around but never detonate because it cannot reach the center. The only way to use it efficiently against a factory is to target a nearby structure, or to use attack ground (heavy micro required).
2) it's hard to use because one firebeetle will kill all firebeetles around it if they are near. Mercies don't do that.
3) it's hard to use because there is no button / shortcut to make it instantly explode. If you ctrl+k it, it will die without doing damage.
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Now, here are some workarounds and possible improvements:
1) Stats improvements :
MaxRadius = 5 (from 3); it's the range at which the unit will detonate from its target.
DamageRadius = 3.25 (from 3); it's the AOE.
FiringTolerance = 180; it's something to make it detonate without having to look at its target in the eyes (or so I believe)
In order for Firebeetles to properly attack structures and units of all sizes, without having to be exactly in contact, it needs a range attack of 5 (between 4.5 and 5). But then it also needs a slight AOE buff to 3.25 (that's the smallest possible value, otherwise, it detonates too far from its target when it has a small size). And you don't want it to miss the ACU you are hunting.
2) it's possible to render Firebeetles immune to other Firebeetles explosions.
3) it's possible to add a button in the unit panel to instantly detonate it and make the ctrl+k command as an instant explosion dealing damages.
I'm uploading here a previous version of the Community Balance Patch. Anything not related to the firebeetles is irrelevant (3603 came,etc). It's just for you (especially the Balance Team) to test these features. Brute51 (I have his authorization) coded a new weapon behaviour for the firebeetles that improve it a lot while not touching much the stats of the unit (damages, HP, speed, etc). I adjusted slightly a value because I realized the damage radius becomes too small if you increase the range, and then you miss your target.
If the new features (additional button and shortcut, immunity of firebeetles to the explosion of their brethren) seem to the balance team unecessary, please still try the mod to test new values and see the difference when trying to attack factories and targets that are slightly too far away resulting in the complete waste of the unit even though it managed to reach its target (and somehow failed to explode, as seen too often).
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Just unzip the file in your mod folder. Both mods are required because teh community balance patch required the community bugfix patch, but don't bother with all the stuff that isn't firebeetles related.
If you like the ideas above, especially the ones that require Brute51 custom code, it would be possible to add them in FAF, or something of the sort.