Misfire and fumbles

So I got no idea how it works, does misfire stacks with fumble, or replace it ? As if I roll a 1, is it a misfire AND a fumble, or simply a misfire ?

I'm not sure where you're getting any of this.

If you roll a 1, you miss. The rules as written include nothing about misfires or fumbles. Page 240, Automatic Misses and Hits.

Anything else is a house rule

Misfire as in firearms, I guess.
Depends what you call a fumble.

1) Nat 1 is an auto miss, no matter the weapon.
2) "If the natural result of your attack roll falls within a firearm’s misfire value, that shot misses, even if you would have otherwise hit the target", as per the rules.

So, that's two reasons to automatically miss, and I don't see how it matters whether one takes precedence as they're not contradictory.
You miss, your gun's broken, that's it.

Now, houserule about actual, more punishing fumbles aren't uncommon by any means, but like any houserule they mean only those who devised them can explain their effect to you, and we're not them.
In other words, if that's the case : ask your GM.

Pretty sure there's no firearm misfire value in Starfinder.

Wow, was so distracted by the question I didn't even remember which forum I was in. My bad.
Be more mindful when switching boards, I should.

With that in mind, you are indeed correct : there are no misfires in SF.
Either applecat made the same mistake, or that's one more houserule they'll have to get straight.