Thank you for all the work you put into this cachorro8urubu!
It looks like Gen Con 2018 had the same negative effects on the Starfinder Armory, as Gen Con 2017 had on the Starfinder Core Rulebook - very short editing time, as it "had" to be ready for Gen Con...
How does the price of the foam grenades compare to the incendiary grenades?
If roughly the same, the foam should automatically put out the fire, as it is a very niche application imo.
Let's see... the prices for the Foam Granades are as follow: mk1 (lvl3) 410; mk2 (lvl9) 3,520; mk3 (lvl15) 26,100.
At the same time, the prices for the Incendiary Granades are: mk1 (lvl2) 375; mk2 (lvl6) 1,040; mk3 (lvl8) 2,800; mk4 (lvl12) 9,380; mk5 (lvl16) 44,00; mk6 (lvl18) 108,800.
Comparing the ones I've considered in the same tier, we have
Foam Granade...............Incendiary Granade
mk1/lvl3 410...............mk1/lvl2 373
mk2/lvl9 3,520.............mk3/lvl8 2,800
mk3/lvl15 26,100...........mk5/lvl16 44,00
The only major diference in pricing is on the third tier, but it is consistent with price gaps between lvls 15 and 16, and it is consistent with my assumptions of mk1 foam negating 3 damage (negating 75% of mk1 incendiaries), mk2 foam negating 9 damage (negating 100% of mk3 incendiaries and lower) and mk3 foam negating 15 damage (negating 100% of mk3 incendiaires or lower, 90% of mk4, less than 25% of mk5 and less than 10% of mk6).
After all this, I think I would use the rule of decreasing damage per lvl (mk1 3, mk2 9 & mk3 15) instead of negating damage on lower level granades, because the latter method wouldn't allow for any number of foam granades to extiguish a mk4 and above incendiary one, while the former would allow for x2 mk3 foams to garantee the extiguish of x1 mk4 or x1 mk5 incendiary, or for x3 mk3 foam to extinguish x1 mk6 (yeah, in this case you would be spending less credits on foam granades than was spent on the mk6 incendiary (and only in this case, since on the former, x2 mk3 foam against mk4 or mk5 incendiary, you spent more), but not by a long margin, and you would still be using 3 turns/actions to put out that fire that took only 1). Or you could use the dice values I've assign (mk1 foam 1d4, mk2 foan 1d6, mk3 foam 5d6), but I don't think it would fit quite as nicely.