I did, and I disagree (for future reference). The Firebomb entry says "a target struck by a fire bomb must take an Ag test or catch on fire (see [..] p210)". Page 210 refers you to p128, which is the Flame weapon property.
There's room for interpretation, but I saw no reason to think that a firebomb would set you on fire any easier than a flamethrower, so I went with that. Academic as it turns out, though, at least in this instance.
Page 210 also contains the rules for what happens when you are on fire, which I assumed is what the Firebomb entry was pointing at. The only reasons I can think of that a grenade would set you on fire more easily than a flamethrower is if the flamethrower were a gas-jet version (40k ones aren't, Promethium is liquid,) or because the flammable liquid being spewed about is centered on your location and has a higher chance of getting on you specifically. Against most human enemies, it isn't going to matter overly much which way it goes.
And yeah, I think the AP6 is to represent burning liquid leaking between cracks in your armour, getting through weak flexible spots, and generally being extremely difficult to stop.