Can Flash Attack be used with Hand Crossbows and one-handed firearms?
Short answer: No...
Flash attack states:
When wielding two light weapons, you may attack once with each...
Hand-crossbows are not technically light weapons, they are technically ranged weapons. I would guess this holds true for firearms as well. If you take a look at something like a light hammer, you can see it is in the "light" category of weapons.
...But there's wiggle room
Thanks to @hey-i-can-chan for bringing this up. Under the rules for two weapon fighting, in regard to thrown weapons, it states:
The same rules apply when you throw a weapon from each hand. Treat a dart or shuriken as a light weapons when used in this manner
So this presents precedent for weapons being "considered" light even when they aren't in that category explicitly. You can make a pretty convincing argument that hand crossbows should also be given this special exemption as well...
My opinion: Go for it
I don't think it would be unbalanced to allow use of this feat with the hand crossbow. You'd still have to load them after your attack. Even if you use this in combination with the Hand Crossbow Focus feat, it still isn't super OP.
Ultimately, this confusing situation could be because of limitations in how 3.5/Pathfinder classifies weapons into only one category. In 5e, hand crossbows do carry the "light" attribute whereas light crossbows do not (go figure).