...What do you think about firing modes when it comes to multiple attack?...
That's a problematic rules interaction that hasn't been untangled.
Let's lay out some groundwork before I tell you my opinion:
SS: the default attack that incurs no AR/DV modifiers. Costs 1 bullet.
SA: you expend 2 bullets, and get -2AR but +1DV. Costs 2 bullets.
BF: you can do a narrow burst or a wide burst. Either option costs 4 bullets.
---BF Narrow Burst: -4 AR, +2 DV.
---BF Wide Burst: two SA shots
FA: -6 AR, but you get an "improved" version of Multiple Attacks that 1) costs no minor action and 2) can layer multiple attacks on individual targets. Costs 10 bullets.
Now, onto the problems. Indeed, what happens if you do a Multiple Attack to target 2 people with a SA attack? You're either getting -2AR, +1DV per 1 bullet spent on two targets, or you're spending 2 bullets
per target to get those modifications. So, the former option can't be right: SS establishes that you get NO AR/DV modifications if you sink 1 bullet into the attack (on that target). The latter sounds better, right? Surely a SA mode gun that can go BAMBAM can also go BAMBAM, BAMBAM, right? Well, Multiple Attacks doesn't limit you to 2 attacks. You could theoretically attack as many NPCs as you have dice in the dice pool. And, if you're planning on spending edge on Anticipation, you just might. So let's say you're shooting 10 people with SA bursts. The former assumption makes the lulzy argument that you're somehow shooting 10 people with 2 bullets, and the latter says your SA gun is pumping out 20 bullets at twice the maximum rate of fire of a fragging
fully automatic weapon!
It gets worse when you look at BF and Multiple Attacks.
The BF Narrow Burst is "clearly" just a scaled up SA attack. Twice the bullets expended for twice the AR penalty and twice the DV bonus. Everything I just said about SA and multiple attacks is twice as egregious here. But even beyond doubling the egregiousness, is the existence of the Wide Burst, which explicitly is just two SA attacks that have to be assigned to two targets. It's a weaker version of Multiple Attacks, which is fine... but
what happens when you Multiple Attack with a Wide Burst?. We're dividing by Zero here.
And of course you already pointed out the irrelevance of FA if other firing modes can just freely crank out 20-40 bullets per combat round.
So, what's my opinion?
1) That the firing modes and multiple attacks rules are parallel mechanics that aren't meant stack with each other. You want to attack multiple people with a gun? You're "supposed" to use BF Wide Bursts or FA attacks, and those rules give you your parameters.
2) If that's too unfun, and you do combine Multiple Attacks with firearms: then you throw out the firing mode rules, other than the # of bullets you expend. Resolve AR* and DV at the SS firing mode level for that gun, and cap the number of targets you may attack at the number of bullets expended at the firing mode you're using.
*edit: on second thought, probably ought to keep the AR penalty that corresponds with the firing mode bullet expenditure. Otherwise you're making recoil-ignoring multiple attacks, which probably shouldn't be a thing. But absolutely 100% no DV bonus if combining Firing Mode with Multiple Attacks.
Edit #2: I can see multiple attacks and firing mode rules actually interacting if you're using two guns at the same time.