Tell them you'll sign the release if they give you the first 12 months of vesting. Firing someone at 11 months is a dick move. Remember: if you're not willing to walk away, you're going to pay retail (ie, end up with 3.5 months). I'll put odds at 70% that they give you 12 months and get the release they want, and odds at 30% that they give you no months and get no release. That puts the expected return of this approach at 8 months of vesting. In either outcome you keep your self respect (priceless), and the expected return is twice as good as caving.
The key to staying on good terms with them is being respectful and polite while you make your case. If you remain respectful to them, they will remain respectful to you. If you let them walk all over you, you're likely to lose their respect and may actually end up on worse terms with them.
EDIT: Even if you end up with no agreement, your situation still has an option value, a valuation exercise I'll leave to the reader. There's a reason they want that release, and that reason only matters if they become a successful company.
I really think you have it right: negotiate the release. Firing a month before OP's first year of equity vests is an incredibly low move. It's also suggested (and arguably correct) that OP might have legal recourse (the "option") if the company makes it big (which is why founders want OP to sign a release).
They don't want a mess if things get big, so OP has little to lose by offering this deal. There's a good chance the founders accept, too, since (a) they're high on their company's prospects, given their thriftiness with equity, and (b) negative press always sucks, and exercising the "option" would generate negative press.
I'll disagree, though, and say OP should ask for 11 months of vesting--that's what OP earned, after all. It'd be harder to get that extra month, given that OP wasn't there for a full year and the founders have worked to hang onto as much equity as possible.
EDIT: Consult with a lawyer when attempting to negotiate. A lawyer can help you get through the process while avoiding any suggestion of extortion, blackmail, etc. (which, unintended or otherwise, will torpedo you and ruin your rep).
The key to staying on good terms with them is being respectful and polite while you make your case. If you remain respectful to them, they will remain respectful to you. If you let them walk all over you, you're likely to lose their respect and may actually end up on worse terms with them.
EDIT: Even if you end up with no agreement, your situation still has an option value, a valuation exercise I'll leave to the reader. There's a reason they want that release, and that reason only matters if they become a successful company.