The question here is why are you so concerned with keeping on good terms with the founders? This is a humongous dick move, and either they're very shady or they are extremely angry at you. Either way I am not clear why you would work with them again.
But as for the deal, I think it's pretty reasonable given that they are exercising their ability to screw you over. Assuming you signed the boilerplate options paperwork everyone uses, you signed a contract that explicitly stated that you get nothing if you are let go before 12 months. They gave you 3.5/11 of what you were supposed to get. That seems pretty reasonable compared to 0, which is what you are entitled to.
Best of luck. Try not to dwell too much on this and I'm sure you'll land on your feet. As for your employer, karma will surely bite them in the ass if they pull stunts like this and screw over employees. Shame on them.
Burning bridges with the founders is usually more than just not working with them again in the future. Everyone has their own network, and the founders may be good friends with other potential employers, potential co-founders, etc. with the OP.
And if this dispute spills out into public -- "I want to protect the identity of my former employer for the time being" -- then everyone is usually tainted by the end of it. Once it's public, the founders will probably try to justify their decision (by smearing the OP, more or less politely and more or less truthfully), the OP (angry at being smeared) will say something still more stupid, and it will all be preserved for posterity, regardless.
I have no idea what the OP is entitled to, legally or in-all-fairness. But please think through every step very carefully before (and while) burning bridges.
I hope you'll pardon me for making assumptions about your age, but if there were one lesson that I could teach the younger generation (or even just my own child), it would be 'assume good faith'.
Because they want somebody out of the company doesn't have to mean that they're shady, or that they're angry. I can think of dozens of reasons why you might want to get rid of someone that wouldn't have any bearing on them personally.
If there's one lesson that I could teach the younger generation, it's that you ought to trust what a person shows you & tells you about their honor and the value they place on their word. When a person shows you who they truly are by acting like this, don't make excuses for them, don't rationalize it, don't give them a third or fourth chance, and certainly don't "assume good faith."
Assuming the best when a person shows you the worst makes you a rube, begging to be taken.
We're not talking about a friend who lied about why he couldn't come to your wedding or babysit your kid.
You don't get more "bad faith" than firing somebody right before you were about to owe them a lot of money.
When a person dicks you over, the best thing to assume is that they are a dick. This not only protects you in the future, it equips you to deal with them in the present. If you wish and hope that underneath they really mean well, you will only be disappointed and ineffective.
If they were acting in good faith, they would have A: fired him sooner, or B: given him the proportional share of what they originally agreed.
Leaving aside bankruptcy or serious, newly-come-to-light malfeasance on behalf of the employee, the only explanation of a firing right before the cliff, AND a disproportionally low offer (just over 25%!), is bad faith.
Even if they decided he's a terrible fit and they hate him, it's their responsibility for not taking action sooner. They need to fulfill their side of the agreement. Otherwise they are bad actors, acting in bad faith. It's just that simple.
NB: when we fired 2 employees who were not working out at all, I paid them both severance, even though I had zero obligation to do so. Why? Because I believe in acting in good faith. Yes, it hurt my business, but it was the right thing to do.
But as for the deal, I think it's pretty reasonable given that they are exercising their ability to screw you over. Assuming you signed the boilerplate options paperwork everyone uses, you signed a contract that explicitly stated that you get nothing if you are let go before 12 months. They gave you 3.5/11 of what you were supposed to get. That seems pretty reasonable compared to 0, which is what you are entitled to.
Best of luck. Try not to dwell too much on this and I'm sure you'll land on your feet. As for your employer, karma will surely bite them in the ass if they pull stunts like this and screw over employees. Shame on them.