What is the best build for a Firespray-31?

The one you have most fun playing.

Uhm, you'll have trouble finding "the best build" for it. Like many of the ships, its sorta versatile. I'd say the most ubiquitous is just a Bounty Hunter and a Recon Specialist. Gunner is a fine choice too. Krassis with an HLC, Kath with Gunner are popular as well.

With 2 others

Add Rebel Captive, make Phantoms cry.

with Mini-Swarm.

Or Double Firesprays

Or Triple Firesprays (!)

There are many possibilities.

What is the best build for a Firespray-31?

Whichever one I'm using.

a Firespray is really good as a PS 3 Bounty Hunter with zero upgrades. For the cost it is a STRONG contender.

Recon Specialist is a strong upgrade for it if you must add one. Seismic Charge works too, both are cheap.

If you are looking to spend alot of points then it is going to become more and more debatable what you should take. Gunner is often an option, or perhaps HLC. Or Navigator with Fett.... or even Engine Upgrade.

Generally though, simpler is better. 1 upgrade per ship MAX is often a winning strategy.

I am testing out on Vassal

Delta

BH

Vader-EU+Outmaneuver

as long as the green dice are "average" it seems to do well

I tend to run with few upgrades. The Firespray is an excellent ship out of the box. Solid stats and a reasonable movement dial.

The aft firing arc makes lining shots fairly easy.

So far the only upgrade I have done is using higher skilled pilots. But that was a fun list with Fett, Vader (Squad Leader) and Howlrunner...and a wet behind the ears Academy TIE to top it off. Did well against a Han Shoots first fleet repeatedly. But terrible against a GR-75 plus Han, Dutch w/ Ion and an X-Wing support wing. 2 auto evade vs. every attack made it very difficult for 2 AD TIEs to crack shields, and Fett died to a ram. Want to try again though with same list.

With 2 others

I fear this the most

I use one in almost every squad, it's usually just a stock Bounty Hunter, but sometimes I'll add recon spec., rebel captive, or weapons engineer. I use to use Krassis with HLC, but found I was either not utilizing my rear arc because of it, or I was just rushing in close too quickly and then using the rear arc, and the HLC was hardly used, so now I rarely use a cannon. Kath with expert handling is very good, and I think tactician would be pretty great with that build.