Montana is the biggest exception. It's only at-will for a 6-month probationary period, and after that, you can generally only be fired for "good cause".
> A redundancy is different to being fired, and comes with a payout depending on length of tenure with the company.
In US usage, the general term here is “layoff”. And severance is not a legal requirement, and practices vary considerably by industry and, within industries, by individual firm. Layoff is eligible for unemployment, while firing may or may not be. (The line is also looser because the US has at-will employment, so there are very little general rules–other than those in particular employment contracts of unionized or high-status employees–around termination other than for prohibited purposes, or–mostly around warning periods–mass terminations, but there are conditions for unemployment assistance.)
Yeah I know broadly they all are, but some states offer slightly more protection.
On a side note, what happens to health insurance for employees who're let go? Does it end the time of your last day or midnight? I was too afraid to Google it as I likely knew it's be pretty cut throat, but have always stressed about it regardless.
Generally the end of the month that you were terminated in, unless you pay the same company to keep the same plan via COBRA. It's the same plan, you're just paying what you paid for it before plus whatever the company was paying for your share, which is generally much higher.
It might be immediate, it might be end of month, might be some other period. In any case COBRA kicks in and can be paid for retrospectively if you need it.